When you have specified the Starting players of a match, you are also able to add the formation for it.
In the Line-up menu for your team, which you find under the Details tab in the Reporting view, tap on Formation.
You are now asked to choose what kind of formation you are planning to use for your team.
The next step is to make sure the right player is at the right position. When you want to move a player to a new position, simply tap on it.
You should now see it highlighted. The next step is to tap on the position you want to move this player to.
Your player should now be moved. Continue to do this until you have the preferred formation.
Once you are done, tap on the green SAVE button at the bottom of the page.
Congratulations, you have now added the formation to your match.