On the TTL prism, the light seal is applied
to the base of the prism. Some people have applied it direct to the corresponding area on the body where the focusing screen is, but the original method is the base of the prism and this also makes it relatively easier to remove, but "easy" is not a particularly good word! Removing every skerrick of the old foam seal is laborious, tedious, tiring and testing.
A correct light seal kit can be obtained from Asahi Custom Coverings in Japan:
http://aki-asahi.com/store/html/Pentax67/LightSeal/Pentax67LightSeal.php
That kit comes with all the seals needed, in two sets, so if you have a stuff up, you have a second set to fall back on.
See the video on that site to view application of the seals.
Removal of the rear door channel seals is probably the hardest of any job, and personally I don't think it is necessary due to the dovetail design of the channel; I have only seen this channel seal replacement done once on an old 6x7 (not 67) body, and it wasn't an easy task (7-8 hours IIRC with scraping damage done to the channel!).
If you notice streaks or flashes of light on exposed film, don't automatically point the finger at light seals.
Check and double check that you are loading and unloading the roll of film correctly and carefully, so that there is no opportunity for light piping at the top and bottom of the film being loaded. Put your index and thumb finger firmly against the roll as you draw the leader across to the take-up spool; this keeps the roll firm and dramatically reduces the likelihood of invasive light. Similarly, the finished roll should not be removed until the adhesive tab is in place and the film pulled out. Ideally neither loading nor unloading is done in strong direct light.
Do you have the 1969-vintage Pentax
6x7, or the 1989-1990 vintage Pentax
67?