The process is just as tedious as it has been. Back on M25 some folks used an outside editing software program which made the job easier. However, that ability stopped after M25 and there is no editor in sight. So everything must be done in game.
As for a "how-to," I create an excel spreadsheet with rosters and stats and a bunch of behind the scenes stuff. I then go about trading players from team to team and sometimes changing positions to get the best match from current NFL player to the classic guy. That is the hard part. I try to match ht and wt on everyone but lineman, and I also try to be within a year or two of experience. Then it is just the process of changing numbers, names, equipment and ratings. Then a lot of testing. The process sucks because Madden does not allow complete editing in the main menu roster. Once in Franchise, you can edit ht wt face skin tone, basically everything.
The last couple of years there have not been many classic rosters put out. For M15 and M16 I did 2 each year. M17 I ended up doing 3. I will do 1987 for M18 and I am not sure what else I will do, if anything.
You are right, the lack of being able to carry rosters over year to year is frustrating. It makes doing the rosters that much more tedious. I know a few folks on here that used to do rosters but have stopped for that reason, or the lack of editing capabilities.
Not sure if any of that helped. If I can be of any assistance in problems or issues you have working on a roster let me know.