How to find a way to learn to use your other hand

Weight training. The non-dominant hand needs to take muscle tone. Therefore it is essential to lift weight (not very heavy) with one of the hands to strengthen the muscle and prepare it for use.
To practice every day. One can not be ambidextrous overnight. Do you remember how long it took you to learn to write? Well, that is more or less how long it will take you to get used to writing with the non-dominant hand. A lot of practice and a lot of encouragement.
Use a mirror. While you write you can place a mirror. So you can visualize how you write with the non-dominant hand. This way you can correct the position by comparing both hand.
Start by making straight lines and circles on a piece of paper. At first they may look unstable and not very fluid, but with practice they will improve. When the process is remarkable, we can start with the letters, which are no more than curves, circles and lines. That is why it is essential to start with basic drawings.
Write complete and lengthy sentences as you go through the process.
Write the other way around. For right-handers, writing from left to right is most comfortable, but for left-handers it can be annoying. For lefties, it is more natural to write from right to left. Leonardo da Vinci wrote backwards. Although it may seem complicated, it is more fun and easier than you expect.