This is how to make a one-piece bag using a flat resist. This style has the handle felted as part of the bag, rather than pre-felted and attached, like so:
First, I made samples of different fibers to find out how much shrinkage there was and to decide which combination of fibers I liked best. These samples were originally 8x8", and shrank to 6x6" after felting.
A large work surface is needed. I don't have a work table, so I used a large plastic cutting mat on the floor with an old towel on top.
Also necessary: fiber. I chose a Finnish humbug roving and some locks of washed border leicester.
The resist was cut out of an old shower curtain liner. I measured the size of the desired finished bag, added a few inches to account for depth of the bag, and then multiplied by 4/3 to get the resist size.
To start laying out the bag, I put the resist under a layer of sheer polyester and started laying out the fiber along the edges of the resist.
The first layer is placed all in one direction.
Now the locks that will be on the outer layer are placed on top. The locks extend a slight amount beyond the bottom of the resist.
I only wanted the locks on the body of the bag and not the handle, so only the bottom of the resist was covered with locks.
The resist is carefully slid out from under the polyester sheer. The first layer is wetted out in the center, and the resist is placed on top and smoothed down.
Rub a wetted bar of soap over the entire top of the resist, to help the fibers stick. The fibers along the top of the bag can be wrapped over and wetting down, but not along the lower portion of the bag. Then start laying down a layer of locks to match the placement of the other side.
Once the locks are in place, the remaining edge fibers from the other side can be wrapped around similar to along the top. Smooth and wet these down as closely as possible for the best results. Now a layer of fiber can be placed in the same direction as on the first side, again overhanging the edge. Then wet out this side and cover with a second piece of sheer fabric.
Flip the resist carefully, wrap over the overhanging fibers, then lay out fiber in the perpendicular direction, again overhanging the edges. Wet out and flip again. Repeat this until three layers of fibers have been placed on each side, each layer perpendicular to the one below. The handle portion of the bag should have a fourth layer of fiber placed.
After all of the fiber is placed and wetted, add more soap and start working the felt. I worked this one by hand, but it can also be rolled or whatever method you prefer. Particularly work the edges of the resist so that a ridge does not form.
Once the fibers have felted so that the bag can be moved about without damage, make a small cut in the upper area and remove the resist. Now soap can be added directly to the inside of the bag. Massage the bag from the inside, working out any ridges along the edges of the resist.
Continue felting more vigorously as the felt begins to shrink. Continue fulling as you like until the bag has shrunk to about the size that you want.
Now cut out panels to form handles for the bag. Immediately after cutting, put more soap on the cut edges and rub gently perpendicular to the cut. This helps to "heal" the cut edge. Some fibers will heal edges well (like merino), others do not heal well (long coarse fibers in general). Try not to move the bag around too much until the edges are healed. Then you can finish up the fulling process, rinse, and soak in a bath with some vinegar to reset the pH.
After soaking, I stuff the bag with some plastic bags to hold a shape and then put it on the heating vent to dry.
This is the finished bag, the opposite side from the first picture.
The area with locks is textured like this. If you want something that looks more like a fluffy fleece, use raw unwashed locks instead. The cut edges of the locks felts easily into the backing, the tips tend not to felt in as easily.
I actually made a test bag first using a different set of fibers. These locks were longer and the tips tended to remain free of the felt.
This shows the shrinkage of the bag compared to the original resist. Shrinkage amounts depend on the type of fiber, how thickly the fiber is laid out, how many layers are used, and how much you full the bag.