There is a general rule for finding verb stems in Welsh:

If the verb ends in a vowel (a i o u) then drop the vowel
siopa (to shop) siop-
torri (to break/cut) torr-
addurno (to decorate) addurn-
canu (to sing) can-

If the verb ends in -io then drop the o
Gweithio (to work) gweithi-
Gwylio (to watch) gwyli-

If the verb ends in -ed or -eg, lose your ed, break the eg ie drop the ed / eg.
Rhedeg (to run) rhed-
Cerdded (to walk) cerdd-

As usual, there are exceptions:

The stem for some is the same as the verb-noun itself
Cadw (to keep) cadw-

Some stems are iregular
Gwrando (to listen) gwrandaw-

I have a list of some verb stems that can be downloaded from the Download Document page here but it is still a work in progress