I need help with legs and topknot!

Make sure the leg is dry and straightened of course. Thent take a comb and fluff the hair straight out of the leg and THEN start to scissor. You'll have to keep fluffing as you go and go back over some spots more than one. Also the motion you use to scissor hair is different than what you use to use scissores to say cut paper. Ideally you shold train your hand so that only your thumb moves to power the scissors. Be very mindful of the motion of your scissors as well, don't bounce them in and out of the coat. You can practice by scissoring along a solid flast surface. My first mentor taught me to correct my hand motion by having my scissor along the wall in the salon. (I was abd about taking a snip here and there and then pulling my scissors back before going back in to take another snip - no no no bouncing she would say). I looked like an idiot I'm sure but it had the desired effect and taught me to keep scissoring in a straight line without taking a snip here and there. You can practice on a wool duster by the way... I think it's wool. It won't hurt your scissors and it's CHEAP and gives you loads of pratice. That was one of our assingments in school actually we were each given a "feather duster" as we all called them and told to scissor it into either a leg/foot, tail, or an actual dog if we felt up to it. Wish I had a picture of what mine ended up looking like. I made a poodle out of mine and glued eyes and nose and mouth on it then spray colored it pink. Hahaha even in school I was dying dogs before I even knew anything about it I suppose lol. Then I spritzed a super strong colonge on it and it was a "french poodle" LoL.
Anyway the point it is was good practice and I ended up making several of them on my own out of the dusters I purchased on my own at walmart.

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you can do the snap on comb method Aiden suggested as well, it is easier, but you won't get as nice a leg. So be forwarned that if you're going to a fuller scissored look It will look clippered and sometimes no matter how long you leave it if you clip down the font of a leg like that you that spindly chicken leg look. I personally don't care for the look unless I am not going for a scissored finished and only ever use a clipper comb (just skimming) down the outside of the leg just to give me a starting length to make my shapes, IF I use one at all that is. It's a personal thing though of course, I personally like to do the fronts of the front and back legs all by hand so that I can get the desired shape.