1. Open a level / map (or create one)
2. Point to a location on that map that is in your view
3. press T to teleport the test character there
4. Press the "switch game / editor" button (the one in that pic posted above)
5. Boom you are playing the game right there in the editor. Test yer stuff.
6. Profit
7. Ok nothing ever works right the first time so rinse and repeat until you figure it out.
BTW the Sandbox level is the very best place to test things. It's a complete level with a little bit of everything.
(Composer's advice is sound. Follow this short guide and choose to copy the sandbox template:
https://docs.larian.game/Working_with_levels )