By pressing the proper keys of course 
The current layout is pretty much like this
You’ll find modifier keys in there. A bit hard to read maybe, so here a bit of explanation:
The keyboard works with layers. You may already be familiar with those through the Fn key on laptops, and the Option on Macs. Or even the shift and AltGr keys.
The regular layout on the main layer is where I have the letters. The home rows are, on the left: S L N T, and on the right, A I O E. I use the Lshift mainly to get the capitalized letters. I have a Raise layer - the symbols along the top of each key on the key map. That I activate by pressing and holding any of the keys that says Hi (for high). Correspondingly I can press and hold the Lo key access the Lower layer. So you can see I have the number keys under my right hand in a numpad configuration. I just hold Lo and get access to those numbers. Holding Hi I have access to the mouse keys, so I can move the mouse around, press mouse buttons, use mouse wheel (MW), and have arrows keys under my right hand.
There are still improvements to be made, for instance currently I don’t have PgUp, PgDn, Home or End, but since I use Vim bindings in VSCode and similar on VS I don’t really need those.
In the near future I’ll be generating a better keymap for my own usage, based frequencies of Finnish text and the Rhino code base, so I can optimize where the most used characters are and can minimize finger travel and lateral movement need (mainly for the pinky finger). Naturally that will lead to a new key map that I’ll have to learn. I have found though that it takes only a couple of days to get used to something new. Having to spend time learn that is a minor cost when mirrored against the benefits I can gain from this: ergonomic typing and posture for my hands. Something that I think people seriously should consider when they find they do most of their work at a computer. Preventing injury is in the long run cheaper than having to go to surgery or do other expensive recovery - or even having to stop using keyboards altogether.
I also used to think I need my elbows on the desk, but I actually found that holding elbows on the table causes static stress in the shoulders too easily. If you can hold the body so that you don’t start leaning on them then fine, but I see it time and time again that people are leaning on them, causing stress in the shoulder and neck, and that needs alleviation - if I can save myself trips to physiologist for recovery massage then after a few times not having to go a custom, ergonomic keyboard is much cheaper.
I have a track ball here next to my keyboard, I have also access to a mouse, which I now use mostly for gaming, there is a 3D mouse. And this week I moved my drawing display (HUION Kamvas) between the keyboard halves. Now I have a pen with which I can very naturally do things there, too.
Anyway, next step in the mounting rig is to have some kind of arm system whereby I can get the keyboard lowered to around the hip height. That should make it even nicer for my shoulders, since the arms would be almost hanging in their natural position. The design is naturally limited by my ability to create it, and my Dutch background - it should be cheap to make myself, too. And easy.
I’ll keep the thread posted on progress there (:
