My son was a starter and reliever in HS (also played SS). He started this college season as a reliever but now is starting some. I haven't seen his routine at college....but his HS routine was something like this: I'll assume home game where they got to the field an hour or so before game time. This may not be "textbook", but it worked for him
-Stretch a little...jog a bit...and generally goof off for about 15 minutes
-Throw with a partner....starting at 10 yds or so and progressing up to long toss for as far as he could throw....then back down. Total time probably 10 minutes or so
-Throw with a partner....roughly pitching distance but in the outfield (no mound)...no real windup...just throwing (fastballs at roughly 75-80% and curve/change)....probably 5-10 minutes
-Go sit in the dugout or goof off again ....probably another 10-15 minutes
-Hit the bullpen to throw live with a catcher and get ready to go - 10 minutes