I used GTypist but it was a long time ago.
These days there are excellent tools online: ratatype, typeracer, keybr, keyzen-colemak, typelit.io, typing cat, speedcoder, etc etc.
The main approach is:
1. don't look at the keyboard. Print out a layout, better if color-coded to indicate which finger should be used, and put it near the monitor. Whatever happens, don't look at the keyboard, ever.
2. index fingers on the two central keys with the little rubber bit (F and J in qwerty).
3. accept that it will take weeks to be as productive as you were before, but then it will (should) be much easier to go faster.