Learn the basics. Because you will have to change your basic concept of what familiar numbers look and sound like, it is important to start small. Simply learning how to say and write simple digits like 1 through 9 is a great place to start, as all other numbers build upon these basic characters and sounds.
Say the numbers aloud while writing. One of the biggest obstacles you will face when learning about numbers in a different language is that you have to think about them differently on a conceptual level. So, when you write (一) for the number 1, say "itchi" out loud, so that you can associate the (一) symbol with the corresponding word, as well as its number value.
Incorporate the numbers into your daily life. It is not enough to just study the numbers infrequently. You must use the numbers (both written and spoken) in order to fully immerse yourself to the point where they become natural to use. For instance, when making a grocery list, number it with Japanese characters, and say the numbers out loud as you are shopping. This will reinforce what you have already studied in a practical way.
Incorporate more complex numbers gradually. Like in English, Japanese numbers build upon each other to express higher digits. However, the characters, as well as spoken phrases do not use the same patterns as English, which can make learning numbers higher than 10 a little challenging. Try focusing on a new place value each week to fully understand the written values and spoken properties of that place value.