Praise be to Allaah.
The way to memorize it is to persist in memorizing it. There are two ways in which people can memorize it:
1 – Memorizing it aayah by aayah, or two aayahs by two aayahs, or three by three, depending on how long or short the aayahs are.
2 – Memorizing it page by page.
People vary; some of them prefer to memorize it page by page, repeating it until they have memorized it. Others prefer to memorize one aayah at a time, going over it until they have memorized it, then memorizing the next aayah, until they have memorized it all.
Whether you use the first method or the second, you should not move on until you are sure of what you have learned, lest you build on a wrong foundation. You should review what you have memorized every day, especially in the morning. If you know that you have learned it properly, then you can move on.