You get more steem power one of two ways:

1. Automatically once your post matures, you can get 50 or 100% of your earned rewards put directly into your steem power. This will happen promptly after your post has been online for 7 days.
2. You power up. You can take your SBD earnings, convert them to steem on the market, and then click on "power up" in the dropdown menu next to the amount of steem you have in your wallet. This will move your steem directly to steem power.