Nose Blowing 101! - Omaha,NE

Put a ping pong ball on the table and have him cover his mouth with his hand and blow the ball with air from his nose. (He will see it roll when all he does is exhale through his nose.) After he sees how the air makes the ball move you can have a race to see who can make the ball roll faster. This way he will learn how to blow harder, Then when you give him a tissue to blow his nose you can tell him to do it like he "blows" the ball on the table.
Good luck!

Get a Kleenex
Hold it to your nose
Blow out through your nose into the Kleenex
wipe your nose with the Kleenex
throw away the Kleenex

That is how I taught my kids as Toddlers
They were blowing their own noses from about 2 years old.

Mine is four and I still help her when she's boogery. I hold the tissue to her nose and block first one nostril then the other, giving the instruction to blow through each one. Doing one nostril at a time seems to be easier than getting a big enough breath to do both at once. She can do it on her own for the most part now but sometimes when she's feeling really down and out, she'll bring me a tissue and I'll do it for her. She won't be asking me forever so I don't make a big deal about it.