I don't know how young you can worm them with levamisole, so check with the vet.
Not too hard to do, grab the bird and have them facing with their back to you, hold them between your legs, pinning their wings. Left hand under beak, finger and thumb either side and 'pinch' above their lower beak to open their mouth. Squirt wormer onto their tongue slowly allowing them to swallow - they can do this with you holdiing their beak, as long as their tongue is free,
They curse and swear but it's not too bad.
I buy my Marriages from
SPR