As usual, you will need to work with the stem of the verb. A few examples to give you an idea of what to expect:
es fuhr - fahren (to drive)
es sah - sehen (to see)
es aß - essen (to eat)
And also one complete form for the Präteritum of "fahren":
ich fuhr.*
du fuhr.st
es fuhr.*
wir fuhr.en
ihr fuhr.t
sie fuhr.en
So, as you can see, the personal endings are identical to those used in the regular Präteritum. What‘s different is that there‘s no more -te- involved and that the stem-vowel is likely to change. The problem is that this change is unpredictable. So, again, you will have to learn these forms by heart. But, as soon as you have learned what the change is, the irregular Präteritum behaves pretty regularly, as you might have already noticed above.
A few Rebel Verbs
At times the verb changes beyond the stem-vowel like in the following examples. Can you already guess their infinitives (solution in the footnote** below)?
es ging
es erschrak
es kam
es war
es zog
es stand
These few verbs above are more or less all (base)verbs of their kind. There are still a few more peculiar verbs that we will have to deal with in a few moments but those are almost systematic and therefore do not fit into this category.
** gehen > to go; erschrecken > to be frightened; kommen > to come; sein > to be; ziehen > to pull; stehen > to stand
This topic is covered in: B1-L09