I just finished replacing one of my wrist seals - I ordered these:
Dry Adhesive Drysuit Latex Wrist Seals - Dive Right In Scuba - Plainfield, IL
Mike was very helpful in allaying my fears.
Step 1 - ensure you have some MEK, an iron, a roller (I used a wallpaper seam roller), a heat gun (I have a Wagner heat gun with med and high settings - I used med), a form to keep the sleeve filled while you work (I was able to use a steel water bottle) and some sharp scissors.
Step 2 - turn the sleeve inside out - trim the old seal down to about 1/2 inch remaining. insert the form and pull the neoprene 'sleeve' from the end down along the form so it doesn't get in the way.
Step 3 - plug in the iron and set it to medium. Clean the remaining old seal and area the tape will contact with MEK - use a lint free cloth - then clean the tape as well (I trimmed the tape so that just a small area would overlap - they provide plenty to ensure you have enough). Use the paper backing that comes with the tape and iron the tape onto the old seal area. I did a fairly good job of keeping the tape on something (the old seal or the factory tape) work slowly - as you go, the tape adheres to the suit and it get easier - make sure you ONLY iron on the backing paper so you don't mess up the tape. You can use the roller to ensure it's mostly adhered - remember you will re-heat when you apply the seal, so it'll be ok.
Step 4 - wait at least 10 minutes -then clean the new seal where it will adhere (remember it's inside out) - clean the tape where it will adhere as well. Then place the seal over the tape on the form (my water bottle, your whatever). It will stay in place fairly well, just don't mess with it too much.
Step 5 - I got my heat gun and set it to medium - use a circular motion so you don't burn the seal - heat it up a bit at a time - I used the roller fairly often to ensure things were seating together well. I slowly worked my way around the entire seal - using the roller every couple of inches. Then I went around again - and then again - I really wanted to ensure I had a good seal.
Step 6 - put everything away - let the seal cool - wait 10 minutes or so - turn it all rightside in and check the seal. I put it on and used a bucket full of water to ensure no leaks.
Total time - perhaps 30-45 minutes
I also ordered wrist seals that were appropriate for my wrists - so I don't have to trim! Bonus!
Final step - use some tribolube SC and keep using it (or seal saver or whatever you use to preserve).