Hopefully much of what is covered here will be of use when replacing the seals in other camera models from other manufacturers also though it certainly doesn't cover rail seal replacements that often crop up with various camera types.
Should you spot any obvious errors in the following or have better solutions to share please do email me at the address provided at the bottom of this page.
The first thing to consider is the materials you'll need to do the job and in terms of the light baffling foam itself I heartily recommend purchasing a kit from eBay seller interslice. I buy all my material from this seller and due to the high quality of service provided I have absolutely no inclination to spend time looking for any other source.
Whilst waiting for your foam sealing kit to arrive you may also wish to piece together a few simple 'tools'. I use a surface to cut on (ie: one that can sustain damage from sharp blades with no concerns - some thick card or whatnot), a metal rule to measure and to guide a blade when cutting foam, a sharp craft knife, some white spirits or lighter fluid (depending on my mood!) and some disposable cloths. The interslice kit also comes with a particularly useful pre-cut bamboo tool that proves great for all manner of tasks but in the absence of this you may wish to put together some things like tooth picks etc. I'm not too sure what would work best as the bamboo tool has seen me through some pretty gory reseals so I'm sold on it as THE tool for the job.
Open the film door and avoid touching the existing light seal material if it's obviously in a state of decay (even if it isn't obviously in a state of decay you may find yourself with some thick dark gooey gunk sticking to your fingers upon touching it). If you detect any perished foam sticking to the actual body of the camera carefully remove it with a small piece of cloth dipped in white spirits or lighter fluid.
Unless the camera has been resealed in recent years, and resealed properly at that, you're almost certainly going to have to replace the exisiting seals even if they appear on first viewing to be fine. The latest 500G I came by had the original seals in place and they looked perfect, so I closed the camera door again. Later I opened it to put a film in and saw that just this brief exposure to air had triggered breakdown of the old seals and parts were coming off and sticking to the camera body.
Using the bamboo tool from the interslice kit or whatever you have chosen in its place gently probe the exisiting foam and remove any that easily comes away (the first couple of times I resealed a 500G I went straight in with the solvent and ended up creating enough crude oil to fuel ... anyway). If none comes away easily on brushing it with the tool then it seems you don't actually need to replace the seals.
I use either white spirits or lighter fluid, depending on my mood (do I want a white spirits hangover when it's too cold to keep all the windows open? Can I afford lighter fluid over the former option that week? Do I want to refill my Zippo or am I happy using generic plastic disposables a while longer?) and flood the rear door a little to ensure all the remaining seal material gets a good soaking.
This step probably takes the longest out of them all and is, I have to say, my least favourite step BUT it has to be done and it has to be done well in order to ensure the longetivity of your new seals, particularly in the first instance that they get the best possible grip on the door surface.
Using the sharp end wedge of the bamboo tool scrape away at the remaining seal material. It is particularly useful to get to the underlaying sticky tape like layer as upon lifting this you can usually remove larger portions at a time.
Having cleaned away as much of the old sealing material as I believe I can with the bamboo tool I tear up a disposable dish cloth into small strips ready for finer cleaning of the remaining seal deposits.
Clean up your working area and prepare to measure and cut your new sealing material. I prefer to use 1mm or 1.5mm open cell foam (from the interslice kit).
Once you have the right dimensions remove the paper backing from each piece, liberally lick the sticky back to delay its sticking power for a minute or more, and position in place, sliding about a little as necessary. The bamboo tool is particularly useful for placing the latch end seal correctly. The hinge end seal should curve around with the moulding of the door towards the lip of the door itself.
DO NOT place the door top seal yet.
If you have any suggestions for better approaches or spot any errors do please contact the author at firstname.lastname@example.org