Your puppy should already know the sit command.
Ask your puppy to sit, give him praise when he does. Use a voice-que and hand signal to ask your puppy to stay.
Count up to 3 and give your puppy a food treat and some light praise while he is still in position. If he moves out of place, stop rewarding him and ask him to sit again.
Keep repeating the first 2 steps, slowly building up the amount of time your puppy stays sitting for. Try and count up to 20 while he is still sitting. If he moves out of position, start from the beginning.
Once your puppy can remain in position for 20+ seconds, repeat step 1, but this time, take a small step to the side.
Training can be exhausting for you and your puppy, if you feel progress is slowing down, consider taking a break.
If your puppy has stayed reliably in position while you make your side steps, try walking all the way around him. This is a big step, so make sure to give him lots of praise and treats if he stays in position.