The first time you run Yoshimi you will see the main window (shown on the index page) and also this virtual keyboard to the right.
If all has gone well, clicking on one of the keys should immediately produce sound, and if there was a MIDI keyboard plugged in that should also have been recognised and be playable. The virtual and real keyboards work quite independently, although things can get a bit confused if both are on the same note!
Provided this window has focus, key presses will also produce sound. The layout is shown here for the standard QWERTY keyboard.
You can operate the various controls with a mouse at the same time as playing on a keyboard. Some for the controls are real-time i.e. take immediate effect, while others only take effect on the next note played.
To the left of the keys, there is a slider for pitch bend. Unlike a real bend control this doesn't spring back when you release it, however, right-clicking the control will return it to the centred position.
Alongside and underneath the keys is a spinbox to select the channel to send to. All parts listening on that channel will respond.
Next is a slider to set the velocity of notes being played, and then a rotary control to add a randomising element to make it sound more interesting.
You then have a counter to decide what octave to centre on, immediately followed by independent octave shifts for the key rows.
After the usual window closing button, there is a menu for selecting which MIDI CC (continuous controller), the final slider will emulate.