khan academy is pretty great for learning math on your phone.
Learn at your own pace and free!
Learning math comes in basically two components: lecture and practice.
During the lecture phase, you’ll need to set some time aside so that you can absorb the information. In the past, I’ve had good experiences with Khan Academy. They break concepts up into small enough pieces that are probably 5-10 minute segments.
After you’ve learned how to solve a problem, you need to practice to keep that knowledge alive. I’d suggest making a ritual that only takes a few minutes. For example: before logging off your computer, you’ll solve five problems of the type you’re learning. Once you get very comfortable and those problems no longer pose a challenge then you’re probably ready to move on to the next concept.
Repetition really is the key here. You could think of it like building a muscle. You don’t get stronger by lifting 100kg one time, you get stronger by lifting 1kg one hundred times. Math is the same way, and cramming for several hours a few times yields poorer results than steadily practising your skills. The best part is that if you do have a busy schedule, you only need to carve out ten or fifteen minutes a day to slowly improve.
First: determine how much time you are able (and willing!) to invest, be realistic about it. Second: see if https://www.khanacademy.org/ provides the math level you have in mind and proceed at a steady weekly pace.
Math is a very broad term. What level of math do you want to learn?
I last left off at calculus 1, I want to learn enough useful day to day algebra and geometry/trig, but also matrix math for game development.