Tell someone what to do
For du, usually use the verb stem without du: komm! Some e-verbs change to i/ie: lies! For ihr, use the usual ihr verb form without ihr: kommt! Formal requests use verb + Sie. Sein is irregular: sei, seid, seien Sie.
Understand the idea
An imperative is a direct instruction, invitation or request. Its form depends on who is addressed. Du and ihr commands normally omit their subject pronoun; formal Sie commands keep Sie after the verb. Bitte softens an instruction but does not change the required verb form.
Build it step by step
For many du commands, use the stem, sometimes with -e. Strong e→i/ie changes often remain, as in lies, but the present-tense a→ä change normally does not. Ihr commands use the usual ihr form without ihr. Formal commands use the verb followed by Sie. Sein has the special forms sei, seid and seien Sie.
See it in a sentence
Sei bitte leise!
Kommt bitte herein!
A useful memory aid
Learn the three address patterns together: one person informally, several people informally, and formal address. This prevents mixing du with an ihr ending.
A common trap
Do not simply remove du from every present-tense form. A command such as lies differs from du liest, and sein has its own imperative forms.