How do you activate a mastery in a game on league of legends?

Answer 1

You have to choose your masteries before the game starts, in champion select.

Do you mean to place a point in a mastery tree, or activate a page of masteries before a game?

>> to pick masteries: home page > masteries.... pick which "tree" you want to go into (follow some good guides on different roles, like attack damage carry, jungler, AP carry etc. Save the page! You can also label your different mastery pages to keep them organized for a quick pick at champion select time.

I like the champion guides on solomid.net (can generalize masteries from one champion for a given role) but you can look at some guides on mobafire.com as well).

>> to pick a masteries page: Whichever page you were last tinkering with on the masteries page will be the one that is "selected" when you go to queue for a match. During champion select, there are the rune page and mastery page drop down menus (to the left of your Summoner Spells). Pick which one goes accordingly with your role!

NOTE: you CAN change/add points in your masteries during champion select, but CANNOT change out runes on your rune pages)

This stuff also applies for Rune pages, except go to Runes instead of Masteries tab at home page.