Hi! I live in Japan,and I'm a native Japanese. 〜ました means it already happened. なります means it is going to happen. But it sometime changes in some case. I know it's complicated. It's difficult learn Japanese even for us! Haha I think your way of study is nothing wrong. Go for it!

In my opinion, there is no "wrong" way to study, unless you are finding that your method doesn't work well for you. Find what works for you best and stick with that method. Everyone learns in a different way. Based on my experience, though, the best way I've been able to memorize anything is by using it. Try out new words and phrases in conversation and if you make a mistake, don't worry, you'll improve as you practice.