-er, changes it to é (manger = mangé)
-ir, take off the r (choisir = choisi)
-re, take off the re and add a u
Certain verbs (-cer and -ger verbs) are stem-changing in the nous form of the present indicative; these are stem-changing the simple past also. The stem change that applies in the present indicative is retained when conjugating the je, tu, il, nous, and vous forms of the simple past. That stem change is reversed when conjugating the ils form of the simple past.
For irregular verbs whose past participle end in -u, (but not for regular -re verbs, whose past participle also end in -u), that past participle is also the simple past stem.
To conjugate in the past tense, add the appropriate ending to the past participle. There is one set of endings for irregular verbs whose past participles end in -u ("irregular endings"), and another set for all other verbs ("regular endings").