I dont know what controls are what for R6S but you could change around the keys. I usually change game controls to suit my left hand, e.g Overwatch: I set mic chat to C instead of ' as it was awkward to use. My thumb sits on C my baby finger for SHIFT / CTRL then WASD controlled with 3 inner fingers. Just play games and it will eventually come to you.

i would set custom binds to make you more comfortable with. like for dayz i have all my stuff on my actual keyboard not the numbers and have them in areas around wasd.

I'm not sure what key bindings are used for Rainbow Six Siege, but I would honestly recommenced setting custom key binds. You have WASD already down, so maybe lean left can be Q and lean right can be E. Crouch I normally bind to CTRL if it's not there already. It's okay if you're not used to keyboard yet, not everyone will get it as fast while others will get it faster; everyone is different, just like some people take months to get used to mouse movement. Example, back when I played Xbox, I honestly thought playing with a mouse for FPS games was going to be a nightmare because of the free motion you have, I tried it out and at first it was a nightmare but I got a hold of it quickly, so much so that thinking back now, I don't know how the hell I used to be able to play any FPS on a controller and how I actually did good like that.

I would just bind what you're comfortable with, if you need practice with key bindings, I would download TF2 since it's free or use CS:GO if you have it, bind the keys to what you would use in Rainbow Six (Leaning isn't in TF2 or CS:GO) and jump into an online match or server browser for custom game modes so you can practice while not being bored of just using single player and doing that stuff. I never used CS:GO custom servers too much so I don't know what they're like, but I think TF2 would be the better one for this practice as TF2 custom game modes can become hectic with boss battles and just the fact how the classes work, I think that would give you a little more of a sense of urgency to learn the key bindings so you don't get killed.

If you have a mouse 5 and mouse 4 key, i say set crouch to mouse 4 and lean to mouse 5.

Banff-_- wrote If you have a mouse 5 and mouse 4 key, i say set crouch to mouse 4 and lean to mouse 5.

Same, I bind useful things to my extra mouse buttons like crouch and push to talk etc... For FPS games I also set the 'melee attack' to the middle mouse button (click not scroll).

Adjust your keybindings to how you like them and whichever is easiest to use for you.

Everyone has their own unique playing style, on all COD series I was 1 as the sprint/hold breath button, up and down scroll as the weapon change and the scroll click as the toggle stance.

I switched mainly to pc gaming after BF4 came out, and it was my "staple" in becoming comfortable with playing with a keyboard. Now every fps I play I customize it to match that of BF. When games use V to melee or F for use/interact its really weird/awkward. I think Arma 2/3 has a great scheme too.
But yeah, keep playing and it will come to you, some sooner than others. Find a good RTS (CoH2 *wink*) so you have something to wind down with if you ever get frustrated with and need a break from mashing keys