Sit is more command than a trick. It belongs to a set of bases to learn with dog to really get along with him. Repeatedly sit is the output command at working on interesting tricks. To learn sitting down you need a dog treat and attention focused on us.
We stand in front of the dog and guide him to sit down with delicacy. We take our hand over the muzzle slightly to the back of his head. We have to keep hand at such height that dog, trailing behind delicacy, puts down rear part of the body.
As soon as the dog sits, praise him and give delicacy. Repeat several times. When the dog begins to understand the movement of the hand, we introduce a verbal command. It may happen that the pup will be figuring retreated, approached from the side. Then try to put it back to the wall, so he is locked from escaping.