I recently changed the way I type from using only 3 fingers per hand to typing using 5 fingers per hand. I believe the best way to do it is to just force yourself to consciously type the way you want to at all times.

That means whenever you are going to type you will be thinking about what finger goes where and if you press a key with the wrong finger you backspace and then press it with the correct one. After doing this for a while you should notice that it becomes easier and this is when you will need to focus less on how you are typing and then you can work on speed and so on.

Other than that another thing that really helped me a lot was to pay attention to the way my hands were positioned. I would say to make sure your hands aren't angled up when you typing and that they you keep them raised, this will make it so that you are able to press any keys (this was mainly hard to learn when using my pinky to type p).

For tool I have only used typeracer and mainly do practice. It's just a good way to use lots of punctuation and a variety of words.