There are a number of youtube videos you can take a look at:

https://www.youtube.com/results?search_query=apply+pinball+decals

There are also several restoration threads on the forums detailing the process. For professional level jobs, I'd suggest some of the threads posted by @high_end_pins.

Very briefly: remove/sand off the old artwork, fill in any pits in the wood, stand, prime, (repeat previous steps if needed), paint, apply decals, and trim.

For applying, there are wet and dry methods. Each method has some pros and cons.

If you have questions about certain portions of the process, feel free to ask

Cabinet preparation is 90%, decal application is 10%.

Do the coin door decal first, then the sides. Pre-trim the white excess from the decal from the side that would be the cabinet base as this will be your method for alignment. The top doesn't matter as much as this will be covered by the lockdown bar and siderails.

Dry method works for me as a one person operation - align at the front and then peel off one inch of the backing sheet at a time smoothing it out as you go. I have done wet method and even Chris at HEP does a light mist before applying but for some OCD people like me the temptation to keep adjusting the decal becomes too great and you'll end up screwing up the adhesive by constantly moving it back and forth. Others might disagree, but align it right before sticking it down the first time with the dry method and you won't have to adjust it.

Next gen is a much thicker (durable?) decal material than some of the other alternatives and any minor imperfections in the cabinet substrate are nearly always invisible as a result.

Make sure you use a new, sharp blade to trim the edges

I always liked John's method with the rapid tac, soft clamps and doing one side at a time.