When your dog is standing, hold a treat in front of their nose. Slowly move the treat back (your dog should follow it with their nose) until the dog is sitting. Mark with a click or "yes" as soon as their butt touches the ground.
Repeat a few more times until your dog is comfortable sitting with the lure.
Use your fingers (with no treat) to lure your dog. When they sit, mark with click or "yes" then give the treat from your other hand.
When your dog is sitting almost every time, you can add the cue, "sit," right before their butt touches the ground then mark as usual.
Repeat, slowly removing the lure until you can just say "sit" without moving your hand.