Enabling and choosing it

Android keeps a list of keyboards you have allowed, and one of them is the current one. A newly installed keyboard is on neither, so there are two steps, once.

1. Enable it. Tap Enable keyboard at the top of the app. It opens Android’s keyboard list; switch Opensolr Keys on. By hand, the same list is at Settings → System → Languages & input → On-screen keyboard → Manage keyboards (the wording varies a little from one phone to the next).
2. Choose it. Tap Choose keyboard in the app. Android shows the keyboard switcher; pick Opensolr Keys. From now on it comes up in every app.

To use another keyboard for a while, open Android’s keyboard switcher (the keyboard icon in the navigation bar, or press and hold the space bar on most keyboards) and pick it. Pick Opensolr Keys the same way to come back.

Opensolr Keys hides the small bar with the keyboard switcher that Android draws under some keyboards, and draws its own chevron instead, which puts the keyboard away.