As always, the first step is to find a treat your dog is really willing to work for! Once this treat has been identified, break them up into small pieces (no bigger than your pinky fingernail).
Essentially when you are teaching your dog a command, you will first teach them the result you are looking for (in this case, sit) and once that is learned you are putting a verbal command to it. So to teach the result of sit, first take the small treat in your hand and slowing move it from a little bit above your dog’s nose backwards to over their head and towards their back. Keep it close to their head so they can smell it! This motion of moving a treat over their head towards their back will almost always result in your dog putting his backside down so he can get to the treat. Once your dog does sit down, give him the treat immediately! Dogs only have an association period of about 1.5 seconds, so you need to get that treat to him in 1.5 seconds for him to understand that butt on ground=treat.
Do this about a dozen times or so to make sure your dog is making the connection. Once your dog seems to be predicting what you want when you put the treat out, add the word “sit” into the mix. So every time your dog puts his backside on the ground, say sit and give the treat. Do this a few more dozen times and your dog should have it! Try just holding the treat in your hand, close to your chest, and say “sit.” If he sits, praise him and give him that treat as soon as you can.
Do not be discouraged if your dog takes a while to catch on, some dogs are better at learning new things than others. Just keep doing each step over and over until your dog gets the hang of it before you move on to the next step. Some dogs figure this out in minutes (like my border collie mix) while others take a bit longer (just like my chihuahua mix-he took a few days).
Reminder: dogs get frustrated just like we do, so don’t train with your dog for more than 15 minutes at a time. That way, he can retain everything you are trying to teach him and not get so frustrated that he shuts down and you have set backs.