Using present progressive
how can i put the stem ando or iendo to this word "decimos"
First, I'll give you a hint, decimos is the nosotros form of the verb "decir". Are you able to add the correct ending now?
By the way, welcome to the forum!
Just a note: -ando and -iendo are not the stems they are the endings. For example, the word hablar..."habl" is the stem and -ar is the ending
If you go to the "More" button, and then "conjugation", you can type in the verb, in any tense, and it will show you all of the conjugations for that verb. It will include the gerund (ing) form at the top.
For -er and -ir verbs, the gerund ending is -iendo; for -ar verbs -ando. Decir would use -iendo. As you will see at the conjugation page, though, decir is an irregular verb, so a little twist to the rule.