The first thing to do is keep calm! Lay a towel on your lap, stand him on it and wrap it round him quickly and quite tightly (not too tight!) then hold him with his back to you and stroke him and chucky his chin to calm him - have the syringe ready and put it to to the side of his mouth, don't force it in or push the plunger just let him get used to being there then very very gently syringe the medicine into him. It takes patience but like with all animals if you appear calm it helps them, but I agree the first time is nerve wracking!
Hope this helps I'm sure others on here have done it too!
Good luck!!