The best way to do a topknot or any grooming is up on a grooming table (a folded towel on a washing machine makes a great one). He will behave much better this way. Then gradually get him used to having his head worked on, giving him lots of praise and pieces of treats as you go. When he's had enough, put him down and end on a positive note.
It's best to start with just a little band to get him used to the feel of a topknot before adding the weight of a bow.
This is a great topknot tutorial:
Topknots shared by Linda Grimm and Maria Lelinho starring Rocky! Many Thanks!