I have a very cheap crummy 10$ sleeping bag, and I'm adding a fleece liner to make it more comfortable and increase it's functionality.

Because this bag is such a cheapo piece of junk, the materials used actually ended up costing more than the bag. However, if you are an avid camper and maybe have several really nice bags, then this zip-in liner system can be interchangeable, and possibly extend the life of your higher-end sleeping bag. Its all about better living through doing-it-yourself.

The Liner is designed to zip directly into the edges of the sleeping bag, allowing you to use the installed sleeping bag zipper to vent the bottom if your feet need some air. The liner will also still allow you to zip together 2 identical sleeping bags to make one big one, if needed. The liner can also be removed entirely, and zipped up on its side just like an actual sleeping bag.The other advantage to having a sleeping bag liner is that it can be removed and washed, which keeps the bag itself cleaner.

This instructable is made for a standard adult rectangle bag, but it will probably work with a mummy bag as well, with some modifications.
The sleeping bag used here is Purple on the outside, and yellow on its Inside as a reference.

First, Gather Materials:

1 regular adult rectangle sleeping bag
2 open-end separating zippers, 100-inch length each, preferably with dual pullers.
2.5 to 3 yards of polyester fleece, available from a fabric store
Sewing Thread

Step 1: Fabric Cutting

Fleece fabric usually comes in a width of 54 to 60 inches. I found some truly hideous material on sale at a fabric store that is a basic 2-sided brushed polyester fleece. Its easy to sew, machine-washable, and the pattern will scare off any marauders.

I started by laying the fabric on top of the bag folded length-wise down the middle, but this is a little bit too narrow to make a good liner when the fabric is folded lengthwise.
So, you can refer to the photo below where I am turning the fabric to fold it width-wise, with the selvage edges at the top and bottom of the bag. (selvage is the term for the white edges with the printed fabric info on it)
To be comfortable, my liner needs to be about 72 inches wide total, and the length will end up being whatever the fabric distance is from selvage edge to selvage edge; in this case, about 60 inches.

I know that 60 inches is not long enough to meet the length of my sleeping bag, so I am measuring from the selvage of my liner, down to the sleeping bag zipper.
I get a distance of about 18 inches.
So, I am going to trim off the excess fleece material that i don't need, and attach it onto the bottom.

Step 2: Piecing the Bottom

I have trimmed off the extra fabric from the fleece, and now I have to add to the selvage edge to make the liner long enough.

I am sewing 2 pieces of fabric to the bottom of the fleece in order to match the length of the sleeping bag.
It is possible to make a bag liner out of 1 very enormous piece of fleece, but It would take about 5 yards of liner material to do it, and I have decided to opt for economy of fabric for this project.

My liner has a total length of about 60 inches. This is about 18 inches shy of what I need.
The excess fabric I am using will be turned sideways and stitched on to the bottom of the liner. the piece measures 60 inches by 24 inches.
I will use the short side of the material to cover the 18 inches I need, but the long side is 60 inches, and my fleece is 72 inches wide. So I am taking a piece of white fleece I have sitting around and I am going to sew a strip of it onto the side to meet the liner width I need. It will probably look nicer to use more of the same pattern for this strip of fabric, but it looks more clear in the photos to use white.
My Liner bottom piece will be 24 inches tall for now, and the width will be end up being 72 inches including the white strip I have to sew on.

I need to square off my fabric to make sure the seams will be straight, so I am marking it with a sharpie and trimming off the edges using the L-Ruler.

Step 3: Sewing the Bottom

Its a good idea to pin the pieces together before sewing to prevent any seams from puckering .
I'm sewing the white piece on first, then sewing the bottom part onto the liner next.

I'm using a small zig zag stitch to join up the pieces, then I've trimmed off the excess seam allowance down to about a half inch.
The zig zag stitch and satin stitch will prevent any stitches from ripping if the fleece is pulled too much in any direction.

I'm topstitching the seam allowances down using a satin stitch to flatten it out a bit, and keep it from being too bulky. It will be a little more comfortable for sleeping.

Step 4: Deploying the Zippers

Zipper Time:
Open- ended zippers are the type that can completely separate, the same type as used on a jacket and most sleeping bags. The bottom end of the zipper has the parts called a Pin-and-Box assembly. This is the bit that you insert into the zipper puller to get the zipper to connect when you join it together.

The liner zippers will be sewn directly to the sleeping bag; next to, or on top of the sleeping bag zippers.
Start by pinning the liner zippers onto the bottom at the center, with the pin-and-box pullers facing the center bottom of the bag. The liner zipper is pinned 'teeth down' or facing towards the outside shell of the sleeping bag.

It is difficult to sew zippers around corners on sleeping bags, so its important to curve the zippers around the sleeping bag corners in order to sew them on. The best way is to stack them next to the sleeping bag zippers to follow the same curve. I have aligned the zipper at the same starting point as the native bag zippers, I can then follow the zipper curve and pin it along the bag until I reach the top.

Pinning the zippers can also take a long time if your cat refuses to stop chewing them.

Step 5: Sewing the Zippers

Now that the Liner zippers are aligned with the native bag zippers, I can begin sewing. I am starting from the pin-and-box end, and sewing along the zipper tape. I have also marked the center fold of the fleece with a safety pin as a reference point.

I can use a zig-zag stitch on this to make it really durable, but If my sleeping bag falls apart (which is likely), it will be easier to pick the zippers off the bag if I use a regular straight stitch. That way, I can install the zippers into the next bag and my liner will still fit in.

Step 6: Pin and Trim the Fleece

Now we can pin the fleece liner onto the zipper.

Before I start pinning, I need to trim off the Selvage edges. Then, I am going to align the fleece at the top of the sleeping bag, and fold over about an inch and a half to make a nice hem at the liner top opening. I can sew the hem with a satin stitch before I pin it onto the zipper tape. But in this case, I will just fold it over, pin it, and sew the hem a little later.

Starting at the top of the sleeping bag, I can pin the liner fleece onto the zipper tape, until I get to the bottom of the sleeping bag.
Once I reach the bottom, its time to trim off the excess and make sure the fleece isn't too long. I am going to just eyeball the fleece, and trim off about 5 inches from the bottom, so it will align with the liner zipper I have sewn on.
Its also important to remember that the zippers will be curving around the corners, so I will need to trim off the corner points of the fleece.

I'm pinning the fleece to the zipper with the right side of the fleece towards the sleeping bag shell, and the seam allowances are away from the shell at this point. The fleece seam allowances will end up away from the body on the finished liner.

If I had to do this again, I would make a note of the length and the width of the liner, so I wouldn't have to trim off fleece as I go. But my feeling is that all sleeping bags are a little bit different in length and width, and using this method means I will get a perfect custom length no matter what kind of bag I modify.

Step 7: Sew the Fleece

I've sewn the lining zippers to the sleeping bag, and I've trimmed and pinned the fleece to the zipper tape. Now its time to sew the Fleece on to the lining zippers.

It works best to use a zipper foot when you sew on zipper tape.

Step 8: Finishing the Fleece and Bottom Hem

So, now I have sewn the fleece to the zipper tape.
I decided to finish the fleece seam allowance with a zig-zag satin stitch all the way around the fleece.
This adds some extra durability to the fleece, and also stitches down the hem at the bottom of the liner, at the center between the 2 pin-and-box zipper pullers.
I am also hemming the top opening of the liner now as well. I'm using a satin stitch on the hem, and the hem is about an inch and a half wide.

Step 9: Finished Liner

My Sleeping Bag Liner is basically done at this point.
I can zip it or unzip it from the sleeping bag, and I have turned it inside out so that the seam allowances are away from the 'body' side. This means it will be smoother on the inside, and i will still be able to zip up my whole sleeping bag with the liner attached.

If you want an interchangeable feature, you can acquire 2 more identical zippers and sew them into a second sleeping bag. That way you could have a second bag with the same zipper setup, that would allow you to easily zip you fleece liner into it.
You could have a summer bag and a winter bag with a liner to share if you're inclined.

This material made my eyes hurt to sew it. Next time, I will have to find a less ugly pattern.