Take the verb in the yo form of the present indicative, drop the final -o, and add the 'opposite' vowel endings:
- -ar verbs use -e endings: -e, -es, -e, -emos, -éis, -en
- -er / -ir verbs use -a endings: -a, -as, -a, -amos, -áis, -an
Because we start from the yo form, any irregular stem carries over (e.g. tengo → tenga, digo → diga).