How old is your lab. To train a pup it is easy, but then since Labs love food it is easy with an older dog too.
Take a small piece of some favorite treat of your dog and hold it in front of his noise and move your hand back a little till me stand and then give him the command if he sit then again take it to him and then slowly take your hand back. The duration of the stand position slowly increase. I cant explain any further in words, but I hope u have understood what I have tried to say and explain.