Practice your weak letters
Finish a song first and we'll build a practice passage from the letters that gave you the most trouble. There is nothing to set up - the drill comes from what you actually mistyped.
Where these letters come from
Every time you finish a song or a typing test, the end screen counts which letters you actually got wrong - not how many mistakes you made, but which key was under your finger each time - and shades a keyboard by them. The five that tripped you up most become the target set for a drill. It is a small number on purpose: spreading the reps across a dozen letters is how you end up practising nothing in particular.
How the passage is built
Words mode scores every word in our list by how many of your target letters it contains, drops the ones that contain none, and then picks from what is left at random, weighted by that score. Weighted rather than best-first, because always taking the densest word would hand you the same passage every time you came back with the same weak letters. You get 45 words, eight to a line, and a genuinely different set each time you ask for new words.
Characters mode throws the dictionary away. It builds short nonsense words, two to five characters each, from a shuffled bag of your target letters that refills as it empties - so over a passage each letter gets roughly the same number of reps instead of whatever chance hands out. Real words carry a lot of letters you are already fine at; this is the option for when you want the reps on the keys themselves. It needs at least 3 distinct weak letters to be worth offering, because combining fewer than that produces barely any variety.
Why target letters at all
Typing more is not the same as getting better. Ordinary English is heavily weighted toward letters most people already hit reliably, so a long generic passage spends the bulk of its keystrokes confirming what your hands can already do, and gives you a handful of goes at the one key that is actually costing you accuracy. Weighting the passage flips that ratio. It is the same idea as the pause-on-fall-behind in the song player: put the difficulty where the problem is, and leave the rest alone.
More on how speed and accuracy trade off against each other is on the common typing mistakes guide, and how WPM is measured explains why an accuracy problem shows up as a speed problem.