This is the article that jceffali was referring to in the chat. Excellent high-level understanding of how to handle Spring Training!
Spring training is a time to try out some new players, get used to the game, and give your prospects some work. It seems like every experienced owner handles this a bit different, but here is what I do:
Spring Training Roster: Go to Manager's Office: Player Settings: Spring Training Squads. You will automatically have your entire major league roster already. I suggest adding your best prospects (about 15-20 total though can vary) from AAA and AA, and maybe even a top-notch prospect from HiA. I don't tend to use my youngest prospects in ST, but that's just me.
Veteran Players: Apparently there is a penalty of some sort if veterans don't get used enough during spring training, so you want them to get some work. However, there is no benefit to working them full time, as they will tire needlessly and be exposed to injury risk. I usually put my veterans on rest after about 25-30 ABs or 3-5 pitching appearances.
Prospects: Give them as much work as you can, especially the ones with a lot of upside in projections.
Lineup Strategies: There are multiple ways to do this: One is to start with your veterans then put them on rest when they get enough work. Another is to edit manager settings so that it rests starters after 5 or 6 innings. Be sure to set pinch-hitters, defensive replacements, and player rest or it won't work correctly.
Pitching Strategies: Again, there are options. A lot of owners use tandem starters, which allows certainty that those two pitchers get in the game. Another option is to use a standard rotation but put the veterans on rest after they get work in and start prospects later on. In any case, after setting the pitching staff, adjust the player settings for pitch counts.
A Note on Rest: Pitchers get fatigued with every appearance, then gradually work back up to 100%. If your pitch count is above recommended levels they may not be back to 100% for their next start. Batters fatigue cumulatively over the season. If not rested they may stay at 100% for half a season and then start to gradually drop right when the penant race is hot. For that reason, try to give batters occasional rest all through the season. For batters, the durability rating is the driver. If its in the 90s, a player will need very little rest. If its in the 80s, the player may need to rest once a week to stay at 100%. If its in the 70s, even more rest is needed, and a platoon may even be best. Below 70 is certainly a platoon player.