Bankroll management depends on how new someone is. For extremely new players I would suggest playing only small rake, low buyin, 50/50s with a lot of players. i.e. 100 man $1s. I think someone should put in no more than 5% of his bankroll on any single lineup. So if you have a $200 bankroll and are playing draftkings and fanduel, you could do $10 on each of a thursday DK, sunday DK, thursday FD, sunday FD lineup. There is some correlation in your player choices, but there won't be perfect overlap.

Decide who your must plays are and who your great plays are, fit your great plays around your must plays. It's ok to put your must plays (in cash games) everywhere for 3 reasons:

1. other players will have them too so you are at less risk if that person does bad.
2. these are the highest value players and allow you to make the strongest lineup
3. you are practicing good bankroll management and losses won't sting you too bad

Stacking in GPPs is really good, It's not necessary in cash games