In my school (albeit English medium, so who knows how much this mirrors reality), we were only taught short form verbs. It literally is just as simple as finding the verb stem, and tacking on the appropriate ending.
Gareth King probably has a robust explanation of doing this (I know you like his book ;)), but, generally you take the last few vowels off, or change the sound slightly (sort of like strong verbs). So, deall -> deall-, gwrando -> gwrand-, cefnogi -> cefnog-. There’s a few irregular stems, none of which come to mind right now, sorry, but I recall mwynhau being one of those (I thought it was mwynheu, but my dictionary disagrees with me, so anyone else know?)
Then, just tack on the ending, which is pretty much fully regular outside of the 4 irregular verbs. This just involves remembering the tense and the subject. So, deallais i (like in Level 1!
), gwrandodd e, cefnogon ni.
So yeah, just memorise the “irregular” stems, like you would with strong verbs, and then just stick on the appropriate ending.
EDIT: Ok, I dug out my grammar book. Chapters 205-208 describe how to get the stem (but, give yourself a bit of practice and that should feel second nature – certainly not something I ever have to think about these days, thankfully), and chapter 209 gives the exceptions. It’s basically what I said above: remove the final vowel, but, if the final vowel is -au, change to -eu, and -io goes to -i.
It’s also worth mentioning, and easy “cheat” with the irregular verbs, is that bod and mynd are the only “really” irregular verbs. The other ones, dod, cael and gwneud can be found by sticking a letter in front of the conjugations for mynd.
I went - Es i
I did - Wnes i
I came - Des i
I took - Ces i