Step 1: Conditioning the clay
Then you will need to grab some tools to help condition, flatten, shape, and incise into the clay. I used an X-Acto knife, a green plastic knife, a few basic polymer clay modeling tools, and a pointy tool.
After you have gathered all of your materials you will want to condition the clay by cutting the 2 oz package into 4 pieces with the X-Acto knife. Then work each piece separately in your palm until it is softened. Then combine all of the pieces into one ball and keep working them together.
Separate the ball into 2 equal pieces and save one half for another project (or use it with your stamp.) Wrap the leftover clay in aluminum foil because polymer clay will interact with most types of plastic containers and sometimes plastic wrap.
This step is important because it makes the clay soft and mixes up the materials within the clay that makes it strong and non-brittle. Once you condition clay, you will never need to do it again. If you own a clay conditioning machine (pasta machine) you can use that. Make sure not to use anything that touches polymer clay on food though, because although it is non-toxic to use, it is not safe to be ingested.
Step 2: Rolling out the clay
Push down the blob until it is sturdy enough to use a rolling pin on. If you dont have a dedicated polymer clay rolling pin you can use a round pen case or a thick dowel.
Roll the piece until it is about 1/4" thick. Try to keep the clay circular in shape by flipping it over occasionally and rolling in the other direction.
Step 3: Incising into the clay
To make the grain texture I started out by mapping three knot shapes and two pointy triangle shapes that wood grains usually have on various part of the stamp. This helps you make your lines around these features to make the texture look more realistic.
The pointy tool is good for making thicker lines and curved lines. The X-Acto knife is good for straighter, thinner lines. You can make a line thicker by running the pointy tool over the line a few time and also you can wiggle it slightly side to side. Varying line widths helps make the texture look more realistic. Try to keep the lines going in the same basic direction, but have some of the lines start and stop at different points.
If you get small pieces of clay on the top of the texture use the pointy tool to flick it out. If you made a mistake you can push the clay down a bit to correct the error and re-incise into the clay. The knots should be made with the pointy tool. Comma shapes work well for the knots.
If you want the stamp to be a specific shape, you can use a cookie cutter. Also it may be useful to incise an arrow on the back indicating the direction of the grain for when you are using the stamp. If you want to do this after you have baked it with a Sharpie, that is fine as well.
Step 4: Curing the Clay
If you do a lot of polymer clay, having a dedicated toaster oven is a great idea or else your home oven will need to be cleaned frequently because it will pick up the odors of the clay. Also, most ovens are off by as much as 100 degrees, especially toaster ovens, so investing in a cheap in-oven thermometer would be a good idea as well.
Clay needs to be cured at the right temperature and time for a few reasons. If it is cooked at too low of a temperature (or not long enough time) the inside of the piece could be raw. If clay is left raw on the inside, it will slowly begin to degrade the cooked clay on the outside and within a few month to a year your piece will be gummy and ruined. If your oven is too hot (or you cook it too long) you can burn the piece and cause caustic fumes and a burned and brittle piece.
Placing your piece on a piece of aluminum foil make it easy to take out of the oven. I also use a fan to blow the air out of a window because the fumes can give you a bit of a headache- especially if you burn it.
Step 5: Finished Piece
Let the piece cool completely. It will be really hot when you take it out of the oven and it will still be slightly pliable so make sure to put it on a flat surface to cool down. If there are any imperfections in the texture you can remove/ fix them with a pointy tool or X-Acto knife. This is pretty hard to do so it is better to correct as much as you can while it is still soft and unbaked.
After the piece is cooled and you've looked it over it's ready to use!
All you need to do is push the stamp into a piece of rolled clay (like you did for the stamp itself) and apply even pressure over the entire piece. When pulling the stamp off the clay be sure to pull upwards and peel the clay you are stamping off slowly so you don't smudge the pattern. The clay should be easy to remove and the stamp should stay clean after every use. If you do get any new clay stuck into the pattern, make sure to remove is with an old toothbrush and wash the stamp off.
It is better to cut out the shape that you will want after you use the stamp because you might smudge the sides of your fresh clay or distort the shape slightly while removing the stamp from the clay.
The last two pictures show what the stamp looks like once a shape is cut out. This shape is from a woodgrain piece I made for an Instructable that I will post for the Valentine's Day 2012 challenge.
This technique is really easy and you can make many other textured stamps using the same basic steps. You can also make wax seals using the same techniques but on a smaller scale.