How should my family introduce a 3 month old labchow mix to a 3.5 yr old pumchi

You don't say what genders they are or whether the older dog is neutered.

Just watch them. The older dog may not tolerate the usual puppy play which often involves nipping especially around the face, and may growl or bare his teeth. This is expected 'teaching' of the pup.

Give the older dog plenty of love and attention. Dogs do get jealous.

Keep them away from each other's food at first and be there to watch, especially to see what the dog does when the puppy gets near his bowl. Acclimate feeding times gradually.