To earn tiokens viewrs have to link their Battle.net account with their account on Twitch.tv, MLG.com, or overwatchleague.com.
"Each time a map concludes during a live broadcast, eligible viewers will earn a League Token," reads the announcement. "Also, a percentage of lucky fans viewing the live broadcast when a match ends will be awarded 100 League Tokens, enough for a home-team skin of their choice!"
Each Overwatch League match consists of four maps, with an additional fifth tiebreaker map if the teams are tied 2-2. At three matches per day, four days a week, that amounts to a total of between 48 and 60 tokens per week if one were to watch every single match.
Twitch has partnered with Blizzard to reward League viewers with Overwatch Tokens. This is the new type of currency launched along with the League skins, which each cost 100 Tokens (or about $5, if you buy the smallest bundle available). Watching Overwatch League won't get you any skins in a hurry--you'll receive one Token for each "live map finish" you view--but it's a free bonus for those already watching, and a small incentive for others to give the burgeoning esports league a chance.
Twitch is also developing a VIP ticket which unlocks even more in-game rewards, behind-the-scenes content, and other perks. Details on VIP tickets will be announced in the future.