This is how we present nose-blowing to kids in our toddler classroom:
The environment is prepared for their success ahead of time. We cut tissues in 1/2 , to fit their smaller bodies and waste less, and fold them in 1/2 once more. Then stack them in a small basket in a couple places around the home where the child has access to them.
Give a lesson to the child. "Lets go look in the mirror. I see mucous. This is how to clean your nose." You demonstrate by choosing one tissue, holding it over the bridge of your nose, fingers pulling down with the tissue. Fold the dirty part in, and wipe again. Throw soiled tissue in trash, and wash hands. I've seen 20 month-olds enjoy doing this for themselves with success.
When your child has clear sinuses, demonstrate blowing. Place a leaf or feather in your open hand. Show how You can blow air out from your lungs thru your mouth (moving the feather) or thru your nostrils. Another time, practice with a tissue in front of the nose.
Explain "We clear the mucus from your nose by pressing on one nostril and blowing air out the other. Look, mucous!"