Choose a drill...

Most used words (once, in random order)
Drill words starting at rank
Sentences extracted from Project Gutenberg
Drill sentences containing only the top most used words
Drill Length: words (plus the rest of the last sentence.)
Phonetic words that use exactly one starting consonant key, exactly one vowel key and exactly one ending consonant.
Drill Time: minutes.
Numbers 1-99 in simple sentences
Drill Time: minutes.
Other drills.

Finger Drills. Delete (plover v3) or disable (plover v4) all dictionaries but user.json and commands.json (in plover v3, delete all dictionaries and save to get the default config back).