Step 1: Get Treats
I use IAMS shake able treats and cut them into smaller pieces. But u can use any small treat that your dog likes.
Step 2: Use Something to Get Your Dogs Attention
When he was in first training I used that Shake bean bag trainer. Some people can just use a clicker or even there finger. Once they get good at sitting then u should just use ur finger.
Step 3: Make Sure He's Paying Attention
Make sure there's nothing distracting around you. As u raise ur hand up gently push your dogs butt down into a sit. If he sits without a problem then reward him with a treat and praise.
Step 4: Sitting
Your dog will realize that when he/she sits that they get a treat and will do it whenever u do the hand signal. Everytime he/she does it praise them and give them a treat. It takes time but soon they'll do it without a problem.
Step 5: With People Around You
Don't get frustrated if your dog isn't listening. He dosent feel like training right now and just wants to socialize. Even though ur pet should be paying attention to you but don't yell at him/her.
Step 6: Good Luck !
Sorry if this didn't help it's hard to do this without making a video :)