Time to record your own melody using one of the Soundtrap instruments? Hooray! As soon as you create a new instrument track in the studio, it's ready for recording. The way to see this is that the "R" icon in the track header is red. You can play the instrument using your computer keyboard or an external keyboard that you connect to your device via USB (or, if you have an older type of keyboard, a MIDI interface.)
If you're using Soundtrap on a tablet or smartphone, you can play directly on your screen. OK, so now you just have to hit the record button at the bottom of the screen, or press Ctrl+Space, and you'll start recording right after the count in!
(1) Track header - "R" icon (red means record enabled). Record button in the transport section at the bottom of the screen.