I'll take you through the steps to make a sturdy phone cover. I'll be making one for an iPhone 5s but you can make it for whatever phone you have. The cover features a belt loop that enables you to have the cover attached to your belt (Useful if you want to keep your iPhone 6 straight since they have a tendency to bend in pockets).
For a simpler look you can leave out the belt loop and strap/snap-button. This will also shorten the time it takes to make. It should take between 2-5 hours to craft depending on your experience and how many of the steps you skip (If any).

I didn't do everything in the same order as listed in this guide (some of the photos will show) but if you stick to what is written and the notes on the photos it shouldn't be an issue. Enjoy!

Step 1: What You'll Need

The above image shows everything you'll need for this project (Forgot to include compass, pliers, a hole punch and cloth for applying the dye but they are listed on the image). Some of these tools you can do without if you don't have them. For instance the pliers which are only for getting needles through the leather if they are being difficult.

I used 3mm vegetable tanned cowhide, but you can use whatever thickness leather you'd like. You'll just have to make sure you have snap-buttons that will fit with the thickness you choose to use. If you use very flexible or thin leather you could skip the snap-button and instead use a normal button to secure the strap.

Step 2: Tracing and Cutting the Leather

First off you'll want to trace your phone unto the back of the leather. If you don't have the phone available (Maybe it's a gift?) or a wooden mold, you can find the measurements for the phone online and make a cardboard template.

With the phone traced you'll now want to add a bit to it (As shown on the photo) to make up for the thickness of the phone and the leather. First off, add 5mm to the top of the case. This will make sure the leather goes a bit further than the top of the phone (The iPhone 5s is 7,6mm thick - If your phone is thicker than this or you are using thicker leather, add a bit more than 5mm). Then add 1 cm around the rest of the edges. This will make up for the phones and leathers thickness (if your phone or leather thickness varies, adjust accordingly) Do not be afraid to add a bit more than I did. It's also important to keep in mind that 3-5mm on each side will be taken up by the seam.

We also need a strap for the belt loop and snap-button along with a small piece of leather to attach the second half of the snap-button to. The reason why we attach it to a separate piece of leather is to avoid having a piece of metal sliding against your phone on the inside of the cover (This would ruin your day rather quickly).
The size of these parts do not need to be very specific. Just make sure there is enough space for both the snap-button and the seam on the small square patch on the front, and that the strap is long enough for you to add a belt loop on the back (I made the strap 12cm long, with a 4,5cm belt loop). Adjust according to your phone and leather thickness.

I recommend cutting out a piece on the top of the cover to make it easier for you to grab your phone. This can be whatever shape you like, just remember to not go too close to the sides since we need space there for the seam.

Step 3: Neating Up the Edge

If you are like me you may not be very good at cutting curves. As you can see on the photo I cut a rough curve and then sand it with the sanding paper to get a proper round curve.
Next we need to bevel the edge. You can go for a large bevel or a subtle one. In this case I went for a very subtle one.
Beveling the edges help give the project a more professional look once it has been burnished so I don't recommend skipping this step. I also bevel the back of the leather slightly.

Step 4: Dyeing the Leather

Pick the colour of your choice, put on some gloves and apply an even layer of dye on both sides of the leather. Remember to double check the edges - I always forget to dye an edge somewhere.
Getting an even colour can be tricky if you apply it by hand. I recommend working at a quick pace and applying several layers.

Step 5: Attaching the Snap-button

Now is a good time to attach the snap-button (I did this after marking the seam line - No difference).

Mark where you want the snap-button to be and punch the holes with your hole punch. Make sure the snap button fits in the holes (You may have to cut a bit extra with your knife to make it fit easily).

If you are new to attaching snap-buttons it can be a good idea to make a test on a piece of scrap leather, as they can be a bit tricky to lock in proper. You can find helpful videos on YouTube if you don't know how to attach snap-buttons.

I always stress test snap-buttons a tiny bit after locking them just to make sure they don't pop later. If a snap-button doesn't lock proper, throw it away and try with another one (Don't try to re-use broken ones - They won't work). Make sure the leather around the snap-button isn't too thick. If it's to thick the snap-button can't lock, in which case you'll have to thin the leather a bit with a knife.

Step 6: Edge Burnishing

In order to give the edges a nice finish e are going to burnish them. There are different kind of product you can use to help you along with this process, but let's keep it simple here.

Grab some water and dampen the edge (I use a sponge for this, but you can use the cloth you used for dyeing the leather). Let it soak in for a moment and then take your edge burnisher and put the edge into a the groove it fits in (Pick a groove where there is no air in between the leather edge and the surface of the edge burnisher). Now rub the edge burnisher along the edge of the leather at a rapid pace. The water and friction will round the edge and give it a sleek finish. Do this to all the edges. It may take a while to get done but it makes everything look much for complete in the end.

Step 7: Marking the Seam Line

Take your compass and set the points about 3-4mm apart. Place one point
so that it touches against the edge of the leather and the other so that it goes 3-4mm unto the leather. Then drag the compass along the edge of the leather, making a parallel line for your seam to follow. Do this to all the pieces where there needs to be a seam.

Also mark the seams necessary to form the belt loop on the back. You may want to hold the pieces together as shown on the photo to get an idea of where you want the place the seam.

Step 8: Marking the Stitches

Take your fork and use it to mark where you want the stitches to be along the seam line. You do not want to punch through the leather with this, just marking it. You could use a normal dining fork for this, if you have one with appropriate spacing between the points.
Make sure there is the same amount of stitches on both sides of the cover that align nicely (If you have measured something incorrectly it will show now - If this is the case, see if you can't make it fit and if not, remake the incorrect piece).
Don't worry about making the stitches beneath the strap/belt loop and the small front piece, we'll add those later.

Step 9: Making the Holes

Grab your awl and start making the holes you marked earlier. Make sure you pierce the leather at 90 degree angle, otherwise the pieces won't fit together nicely.

Once you have made all the holes following the seam line, place the snap-button pieces as the are meant to be placed on the front and back parts, and with the awl go through the holes you made, marking the leather underneath. Then go ahead and remove the snap-button parts and pierce through the leather proper.
You don't necessarily have to do it this way, but I find it easier since there's less measuring involved and you make sure, provided you go in at a 90 degree angle, that the holes align the way they are meant to.

Step 10: Sewing

We are doing saddle stitches so a needle on each end of the thread is required. Make sure you have enough thread for the seam you are about to sew. I generally take the length of the seam times 3 (1 length for each side of the leather, and 1 length to compensate for the thickness of the leather. If you are using leather thicker than 3mm you may want to use a bit more thread). Remember that it's better to have a ton of thread left over than running out 4 stitches before you're done. If you do run out just tie a knot in between the 2 pieces you are sewing and continue with a another piece of thread.

Start by sewing together the snap-button pieces to the front and back parts of the cover (If you don't do this now you will regret it later, as it's almost impossible to do once the front and back have been sewn together).
When a seam is completed, tie a (double) knot, cut off excess thread and press the knot in between the seam so that it is well hidden (The awl may prove to be a useful tool to poke the knot under the leather).

Now sew the front and back together and then you are done.

Step 11: Stretching the Cover and Final Result

Take a look at your wonderful new phone cover.
If you measured everything right you should be able to push your phone in relatively easy, though it will always take a bit of use for it to be easy to get in and out. If you messed up it'll be a pain and you'll have to stretch the cover a bit. This can be done with your phone, but alternatively you can stretch it a bit with the edge burnisher (Or a similar shape item that wont take damage from being manhandled a bit). Dampening the inside of the cover with water will make the leather more pliable - Stretching the leather along the seams is the most important thing since that's where the most friction is.
Let it dry and repeat until you are confident you can get your phone in there without it being permanently stuck.

If you do get your phone stuck in there don't panic. Leave it be for a while and the leather will slowly stretch and make it easier to get out (You can usually feel a difference 10-20 minutes later, but if the leather is damp I usually let it be until the leather is completely dry).

The photos above show the cover we just made along with my own cover. As you'll notice my own cover is far simpler. I left out the strap and belt loop. The phone is held in by friction and comes out easy enough with a little tap. This simpler design is more pleasant if you want to carry your phone in a pocket.

If you have any questions or concerns, let me know!