Inside the Tactical Suggestions menu, you can press down on your directional button to access the new substitutions menu. It will show your Suggested Subs, Custom Subs, or a mixture of both and can display up to four entries at a time.
Simply press the directional button desired to request that sub. You can press as many as you want, and to cancel them you can press the same direction again. Before going into the menu, you can see the number of subs and sub windows remaining. Note that running out of subs or sub windows will make this menu inaccessible.