Writing them yourself will help get them into your head. Also, I don’t know how many there are for you to learn, but commit to learning two a day. Once you remember a number, you probably won’t forget it. So in a week you could easily have 14 memorized. I tend to think of numbers how they appear on a 10 key number pad, like a telephone, or cash register. I have the pattern in my head, but that might not help much with just 4 or 5 digits, probably just as easy to memorize them straight across. I think I would picture the number as one number in my head. Not sure how this works for other people. Its like one single picture rather than a series of numbers. Kind of like seeing the word cat doesn’t need to be sounded out by most people, we just see the three letters and know what the word is. Not sure I am describing it well.