Item sets let you replace the "Recommended Items" in the in-game store with your own personal build. Set up blocks of items to buy on your first back, another to remind you which items to buy when you're ahead or behind, a third to remind you to buy wards.
YOUR FIRST ITEM SET
Click over to the Items section under the Collection tab, and add your first item set by clicking the plus icon next to the item set search bar (or "Create New Item Set" if you've never made one).
You'll start by selecting which champion(s) will use the item set, and the map you'll use it on. Once you've done that, you'll see the item set configuration menu. You can search for items the same way you do in game. Drag them over to the right to create a new block of items. Use these blocks to save your starting items, or to help remember what you should buy when you're ahead or behind.
FREQUENTLY ASKED QUESTIONS
If I reinstall, will I have to recreate my item sets?
Item sets are saved server-side, so as soon as you create one, it'll be saved to your account.
Can I download item sets from third parties?
You can download item sets from the community, but you won't be able to edit them once they're downloaded.
Can I have more than one item set per champion?