I personally have have the PC version, but I assume the menus are structured the same way.
From the main menu, go to "Help and Options", then the first bubble, "Settings"
Go to the farthest right page, titled "Advanced" and there should be an option called "Auto Land"
Uncheck that option to allow you to manually choose your lands.
Now in the PC version you need to hover over your card you want to play and press Ctrl to choose the lands. In the Xbox version I believe you need to click in the left thumb stick.
I want to play Angelic Edict, a card with a cost of 4
In my hand I have Bloodghast, a card with a cost of which I want to play aswell this turn.
On the field I have 7 lands, 3 Swamp and 4 Plains. Mana pool:
When I highlight Angelic Edict (before I play it) the game will show me (by highlighting my lands in purple) that it will use to play this card, and leave 2 extra white. This is not what I want.
So whilst Angelic Edict is highlighted, I press Ctrl and the highlight swithes to , leaving me 1 white and 1 black.
Still not what I want, so pressing Ctrl again highlights , leaving me with 2 black mana remaining, perfect to play my Bloodghast.