Mobile app
To get to your league's roster settings on the mobile app:
Go to your specific league
Press the settings icon in the top right
Press the tile labeled "Roster"
You'll see your options for roster spots look something like this:
Here, you can use the plus and minus icons to modify the number of each position you'd like to assign.
Flex = WR, RB, or TE
WR RB Flex = WR or RB
Flex (Rec) = WR or TE
Super Flex = QB, WR, RB, or TE
Web app
To get to your league's roster settings on the web app, go to your specific league and click the menu icon.
Then go to Roster Settings and you'll see something like this:
Here, you can use the plus and minus icons to modify the number of each position you'd like to assign.
Additionally, you can hover your mouse over the + Add Missing Position option and more options will appear underneath:
Here you can add Flex and IDP positions.