First the students have to translate 20 Spanish verbs to English. Give each student (or pair of students) two dice - one that is 6-sided and one that is 20-sided (I buy the 20-sided dice at a gaming store - they are made for Dungeons and Dragons, I believe). The student rolls the 6-sided dice for the subject pronoun and the 20-sided dice for the verb. The student must then conjugate the verb with that subject. I have the students record the numbers on the dice next to the conjugated verb (in the squares provided). I also have them include the subject in front of the verb to make the grading easier. Here is what the student's paper might look like:
1 3 yo corregí
2 15 tú seguiste
If you don't have 20-sided dice, you can let the kids choose their own verbs or assign them the numbers yourself. (Sometimes I just have the students use every verb once.)
This pair drill is included in my stem changing verbs bundle. Find it at the link below.
Stem Changing Preterite Verbs Bundle