What this particular exercise does is to take a source text, compile a list of the top 100 substrings for each length 2..5, and then produces a random sequence of them for you to practice. (This is just an experimental idea I am playing with.)
Inside the exercise, you can navigate using cursor keys, skip line with Escape, redo line with Enter, skip spaces with Tab, and return here with Ctrl+x.
It is a simple practice program, and you have to press the keys one after another. That is it.
The idea, for either text or programming, is to load sample texts or source code, and type it out. That trains your fingers not to make mistakes.
It is also a good idea to learn to type using minimal finger movement. Most people use their finger and forearm muscles way more than they need to. This also means that things are tense when keys are struck, which results in many small shocks going into the delicate tendons in the hand and forearm, which is a recipe for injury.
Or choose an example file from the following list.