I have to mention that if these are seedlings they are going to be pretty vigours and you won't get a fruit for many years.
Meaning you'll be better off buying a dwarfish grafted mango tree and keep that in a pot.
Assuming you don't mind and this is just for experimentation or for fun:
Grow them in pots with holes for drainage. (drill hole if needed)
Use potting soil, not the local dirt/sand.
Apply balanced fertilizer regularly
Water it often, more so in summers
transplant them in larger volume pots as the tree gets bigger. (it will probably outgrow the reasonable pot sizes in 2-3 years)
Let it grow in a place with direct sunlight.
That's pretty much it.