When you have created a match and are looking to specify the starting players and substitutes for it, tap on it in the list of matches on the home screen.
You are now taken to the Reporting view. Head to the Details tab.
Under the section LINEUPS, you should see both yours and your opponent's team listed. Tap on your team.
You now get the option to specify Starters, Substitutes or formation. Tap on Starters.
The players in your squad will appear in a list, categorized by position.
It is now just a matter of taping on the ones starting. The green check mark to the right tells you that the player is selected.
NOTE: Just above the green SAVE button at the bottom of the list, you are able to see how many players are currently selected.
When you have picked the right starters, press SAVE.
Go ahead and repeat the procedure for Substitutes.
When you press SAVE this time, the view should look something like this. The two check marks tells you you have now successfully specified your line-up.
Looking to add the Formation to your line-up too? Follow this guide on how to do it: http://help.forzafootball.com/en/articles/3099327-how-to-add-formations