First things first, is your child even ready to learn how to use the potty? It has more to do with ability and willingness than age.
Lets go over the potty readiness checklist:
Can your child communicate effectively with words? Can they let you know that they have to go if need be?
(Keep in mind that in the early stages, your child will NOT tell you of their need. The ability just helps)
Can your child pull his/her pants up and down on their own?
(again, they won't, but, the ability helps)
Can your child walk well?
Is your child scared of the potty? Are they willing to sit and try?
(They NEED to not be scared and willing to sit)
Does your child wake up dry from their naps everyday?
(2 hour minimum dry time)
If your child passed this checklist, congratulations, you are ready to begin the process of potty learning.
I would suggest that before you begin, you set out several days in a row that will be uninterrupted. You let your child know beforehand that you will begin learning to use the potty the next day. At this time, go to the store and let your child pick out many pairs of underwear. The next morning, get your child up and remind them that today is the day. Tell them that it is time to go try to use the potty. After they have sat and tried for no more than 5 minutes, get them up and put them in underwear and explain that they can't pee or poop in them. Explain that they have to come use the potty. Take your child to try every 15 minutes regardless of whether or not they have an accident in between. Do this until it becomes more of a habit. Always say, "Lets go try to potty." before you take them. When they have that down solid with no accidents and are almost to the point of taking themselves...up the time between trips to 30 minutes. Repeat all of the above steps again until they have it down. Then, you can up the time again. Keep upping it by 15 minutes until your child goes alone without your help and has no accidents. Be sure to praise and clap and give plenty of accolades for successes. After about 3 or 4 days, your child should have had an "Ah-Ha" moment that triggers understanding of the function of pottying and how to feel it and realize the need to go. I have seen this method work time and time again. It works based on the consistent uninterrupted time taken to allow the child to learn it without relying on them to understand immediately. There will be many accidents for at least the first day. I also suggest plastic sheets and nighttime learning at the same time. Not every 15 minutes, but, no liquids after 6 or 7. And get them up at least once to potty at night. The key is to make pottying a habit first, then build on that.
Try this out consistently for 4 days at least and let me know how it works for you. You cannot deviate from the learning for the first few days. Do NOT use pull-ups or diapers. Set a timer if you have to. Make sure everyone in the house is on board.
Even a stubborn kid will decide to go if they are being made to sit every 15 minutes.