There is a trend in backpacking called "ultralight". It's a philosophy of doing more with less, minimizing the tendency to excessive features and weight in gear.

An example of the ultralight philosophy is rejecting heavy hiking boots in favor of running sneakers, because they are much lighter and thus permit longer hikes each day. An interesting side benefit is that sneakers while not waterproof are well-ventilated so that when they do get wet, they'll dry out readily, something that waterproof gore-tex shoes have a big problem with (waterproof in also means waterproof out-- they take forever to dry if they get wet, and some argue that in heavy rain any shoe will get wet).

One manifestation of ultralight philosophy is the rejection of rain pants in favor of rain "kilts" (called that instead of "skirt" so men are more comfortable. Not only are they much simpler and lighter than pants, they have several profound advantages:

no human-created sweat moisture accumulation inside because they are totally open and vented at the bottom. Thus there is no need for costly and heavy "breathable" fabrics (which aren't too effective against serious sweat moisture movement anyway when it's raining and 100% humidity and no vapor pressure causing it to move at all)
cooler thus preventing sweating in the first place during high exertion activities. By keeping you drier against your body's own generated moisute, it better preserves the warmth of other clothes and requires less insulation to begin with (don't have to insulate against the chill of being wet)
they can be put on and taken off without removing shoes

Presented here is my design, inspired and improving upon several other homemade designs I found, as well as some commercial offerings from ZPacks, Ultralight Adventure Equipment, Helium Hiking Equipment, Anti Gravity Gear, Mountain Laurel Designs, and others.

Step 1: Features

I examined the commercial offerings and decided to save money and make my own. While I might not have actually saved that much money, having control over the project allowed me to customize and make it have exactly the features I wanted.

Here are the features and rationale for the unique properties of my design:

use thin ripstop tent-fly nylon, not crinkly cuben or not-quite-waterproof silnylon, or tyvek like some other homemade kilts.
employ a conical shape, not cylindrical, to allow legs and knees to swing wide and step up high when climbing large rocks etc, yet not waste excess fabric weight or volume or be puffy, goofy looking, and oversized around the waist. Hiking in the Sierras often involves a lot of high rock stepping.
use of a very lightweight "boning" strip (ask at a fabric store) along the bottom seam, to make that edge of the cone a little more stiff so: (a) the skirt does not billow around in wind as much and (b) it's less clammy against my leg skin when it's damp (a real issue with urethane-on-the-inside nylon and near-plastic cuben; less of an issue with silnylon).
use fabric strips of preinstalled plastic snaps (ask at a fabric store) for the closure instead of velcro or zippers or no closure (which requires additional weight for overlapping material) like the commercial kilts. Velcro loses its stick, catches on lots of natural material, and the hook side can be scratchy on the skin. Having a closure should help in wind-blown rain and/or to block wind on chilly windy nights camping.
use flat elastic with an adjustable fastex closure for the waistband, instead of round bungee material like most of the commercial kilts, because it will be under the backpack hip belt and pressed hard into my bony flesh: important that it be flat and smooth not small and round and pokey.

Now doing it this way took many hours to prototype on paper and then sew (a walking foot was essential on my sewing maching with the slippery nylon).

Materials cost me about $50 but I got way more than I needed and will use the spare supplies for other related items to complete my rain outfit (gaiters, a cape for shoulders, upper arms, and chest).

Step 2: Pattern

Measure your hips with your typical hiking pants (possibly including your insulating layer if you wear thermal underwear or fleece when hiking [p.s. not recommended while hiking but good when motionless at the campsite in chilly evenings]).

This gives the waistband measurement for the top of the cone (the truncated diameter).

Measure from knee-to-knee while you're striding in the longest expected stride. Also measure while stepping up maybe two flights of stairs to represent climbing over a medium-sized boulder or the type of step you typically encounter on improved trails. For example, Yosemite's Mist Trail is full of 1- to 2-foot high steps, requiring a pretty big space between the knees.

This gives the bottom hem measurement for the base of the cone.

You can use trignometry to calculate the measurements of the cone, or tape some paper together and wrap it around your waist, adding bits of paper with tape and removing bits with scissors, until the paper prototype fits well and is a smooth cone.

Lay out the paper prototype on the floor as shown and verify with a tape measure that the top waistline length, base hem, and length are correct. Find the center of the arcs and fix any imperfections with the cone.

Step 3: Seam Allowances

After trying on the paper prototype pattern, Add allowances for seams.

If you're using the same materials as me:

3/4 flat elastic for the waistband. I made a 1" tunnel to allow a little movement of the elastic belt
1/2 boning for the bottom hem
3/4 snaps-on-tape for the door

You'll need to add these measurements to the pattern and include allowances for seams

1/8 seam for the waistband, and 3/8 extra on the inside. This adds 1/8+1/8+3/8= 1/2
boning is wrapped tightly and sewn right on top of the boning (I used sewable boning). No additional seam allowance necessary
The snaps-on-tape tape acts to protect the edge so only a little fold of 1/2 inch is needed. It's trapped under the tape which is sewn along both of its edges.

Thus for my measurements, 35 waistband, 57-3/4 bottom hem, and 21 length, the following are the outside CUTTING measurements (see diagram)

36-1/2 waistband (truncated part of the cone)
60-1/4 bottom base of cone
23-1/4 height

Cut the fabric after double checking these measurements and testing again with the paper pattern against your body.

Step 4: Cut Fabric

I used thin but ordinary urethane-coated ripstop tent-fly-type nylon fabric.

Step 5: Sew Waistband

For all seams, measure carefully and PIN the whole thing together before sewing.

Nylon is very slippery so it all has to be pinned together.

Also-- use a WALKING FOOT on your sewing machine if possible. Until I switched, my stitches got all crazy as the machine couldn't get a good grip on the slippery fabric with just the normal bottom feeder dog teeth and friction against the standard presser foot. There might be other techniques for sewing slippery fabrics like this or silk (add a piece of paper or tape to the sandwich?).

Once the waistband is sewn, leave the end untreated for now

Step 6: Sew Bottom Hem With Boning

This is relatively easy. Just wrap the fabric around the boning. The hard part is getting it to stay when pinning, and keeping it even.

NOTE that the hem is curved so you need a type of boning that is flat but flexible along the flat plane. Not all boning can do this. I used sewable boning (it isn't even called boning, it's a stiffener) that is really a kind of webbing or matrix, making it pretty flexible in both dimensions. If you are using conventional boning, you could create a tunnel like done for the waistband and then just thread the boning through that.

Step 7: Sew Snaps-on-tab

This is where it all comes together.

I used snaps-on-tape, where little plastic snaps are molded directly onto a soft polyester tape material. The tape is easy to sew and the snaps are so close together-- about every 1-1/2 inch, that when closed it's nearly as watertight as a zipper. And unlike a zipper, the snaps will pop open if overstressed and risk tearing the fabric because you took a too-long step or had to jump up a particularly high rock :-)

IMPORTANT: Be sure to install one side of the snaps (say, the male side) on the inside of the skirt while the mating side (say the female side) is on the outside, so when closed, they form a flat system. That means the seams are sewn on opposite sides of the fabric and the tapes are affixed on opposite sides (one tape on the inside urethane-coated side, the other on the outside finished nylon side). To avoid making this mistake, tape a note on the sides of the fabric before pinning and sewing.

NOTE: if you use velcro instead of snaps, be sure the soft fuzzy "loop" side is on the side facing inward toward your legs. The hard "hook" side of the velcro will be scratchy on your bare legs if you're wearing shorts.

Some commercial kilts use a waterproof zipper. That could work, and probably doesn't really need to be waterproof-- the few drops that might leak through a conventional zipper aren't going to be significant considering the entire bottom is open and plenty of drops will bounce up off rocks etc and hit your legs (not enough to make you significantly wet or cold compared to being totally exposed to the rain mostly coming down). If you're using a zipper, be sure it's a detaching zipper (like a jacket) and that you install it so it zips DOWN and thus can be partially opened at the bottom end.

Step 8: Cut Hole for Waistband Belt

Because the ends of the fabric overlap (due to the additional allowances you added to the pattern for seams), and because the waistband cinches down by bunching up the fabric, you'll want the actual belt to emerge from the tunnel a little earlier than the end.

I carefully cut a slit in only the outside fabric of the tunnel about 2 inches in from the end. This length is determined by the size of the buckle you use when closed including the tail of the female end of the buckle that you have to sew down.

The second pic shows a close-up of how the snaps are sewn and on opposite sides of the skirt.

Step 9: Done!

This pic shows one of the advantages of using snaps-on-tape instead of velcro or zippers... if you undo a bit at the bottom, say for more ventilation or because you're taking really big steps on really big rocks and need a little more leg movement room, plastic snaps on tape won't scratch your legs like velcro or zippers can, or catch on fuzzy foliage on the trail like velcro.

Note the nice hoop effect that the boning adds, giving shape to the skirt for ample ventilation, holding rain umbrella-style as far away from your feet and lower legs as possible, and keeping the sticky plasticy feeling urethane away from your skin (if wearing shorts).