From your account’s menu on the main BP site, click Settings. Then click on to the General category in the list on the left.
In there, scroll down to the Reviews section. For Split Reviews, select Yes.
Then, when you go to the Dashboard page (the homepage when you’re logged in, basically), you will see reviews split up into “Grammar Reviews” and “Vocab Reviews”, each with its own review count.
To review only grammar, click the “Grammar Reviews” link. (Unfortunately, while the box containing the link and the counter appears to be clickable, and you might expect it to bring you to the grammar reviews, it currently does not do anything. You have to specifically click on the hyperlink text “Grammar Reviews”. This will indeed start a session with only grammar items.)