For some reason, stock motorcycle seats are almost never comfortable enough to ride on for more than an hour or two at a time. This is fine if you ride a sport bike or never ride more than an hour at a time anyway, but for the rest of us, we need something that is more comfortable than a naked seat pan. If you don't have the money to buy a high-end replacement seat for well over £195, you can always replace the stock padding with better padding, or alter the shape of your seat altogether.
Remove the seat from the bike. This may require tools that are not listed in the materials list, but every bike is different.
Turn the seat over and use the flathead screwdriver to remove the staples that are holding the seat cover over the foam. If the seat cover is held on by glue, remove it carefully so you don't tear the cover. Set the seat cover aside, along with the vapour barrier (thick sheet of plastic) that is underneath it. Don't lose or damage the vapour barrier since this keeps your seat from rotting in wet weather.
Simply lift off the seat pan in one whole piece. They are often moulded foam, so it will be difficult to lay it out flat. You can do the next set of steps without laying it out flat, but if you can, lay the foam flat on a hard surface.
Fix whatever bothers you about the foam. For most bikes, it is the tailbone section that is least comfortable. For these directions, we will be replacing the whole thing.
Lay the foam flat on a hard surface, if you can, and trace the shape onto a sheet of memory foam. Half-inch foam is recommended, but for extra oomph you can use foam a full inch thick.
Use a razor knife to cut the shape out of the memory foam. If the razor knife scares you (and you can't be blamed for that), use an electric carving knife (available at just about any Goodwill or thrift store).
Since the original foam is most likely form-moulded, you'll have to use at least a little bit of it to maintain the shape of your seat (so the cover fits over it).
Use the marker and a small piece of scrap wood about 2 1/2 to 3 inches thick taped to it to scribe a mark equidistant from the edge of the seat. How much of the original foam you want to preserve is entirely up to you. You can always cut away more, but you can't cut away less. Take the plunge and cut away the inside of the seat foam to make a shaped ring. This will help retain the original shape of the seat.
Scribe a line as you did before into the memory foam shape, and cut it just a bit bigger than the traced shape. You need to cut it bigger so you can cut off extra rather than not have enough to fill the hole. Put the ring on the seat pan now, so you can fill the hole without too much trouble.
Test-fit the foam pad into the seat ring, making adjustments with the razor knife until you have a snug fit. Perfection is not necessary, since you will be covering it in the next step.
Take that other sheet of foam, cut out a general seat shape (make it bigger than you need it) and cover the whole thing. A little spray adhesive works wonders to keep the whole thing together. In fact, use a little spray adhesive to stick the insert section to the ring section for more stability, should you have any reason to remove the foam again.
Stretch the vapour barrier and seat cover over the new foam and the seat pad (make sure it's oriented in the right direction) and get some idea for how the seat will look. If there are any low spots or bumps, now is the time to correct them. Fix low spots with small pieces of extra foam cut to the shape of the hole (under the second cover) and glued in. High spots can be shaved down with the razor knife. Work slowly.
Once you've got a general idea for the final appearance, get the vapour barrier and cover and staple it back together. Move slowly and don't bunch up the new foam under the cover as you work. Start by stapling the cover in one spot, then directly opposite that spot, then the sides the same way. This holds the cover in place while you work it over the rest of the seat. If any of your staples need to be adjusted, simply remove them as before (with the screwdriver) and do them again.
Clean up your workspace, and install the seat. A test ride should be at least as long as it took your stock seat to cause discomfort. The is the most gruelling step, but you'll have to ride your bike a lot to test it out.
If at all possible, feel the foam before you buy it. There are many different densities and thicknesses available. It may be a good idea to find an old seat to try this on before you do it to your seat. Some stock seats can be found for as little as £16. Don't worry about how the foam looks while you're working on it; nobody will see it once it's under your butt.
Razor knives can cut through flesh as if it weren't there. Always wear a slash-resistant glove.
Never cut toward yourself with a razor knife; always push the cut away from your body and not in the direction of anyone else. Remember proper cleanup. Even small staples left on the floor can penetrate your tire and cause a flat.