crawl before you walk

To go through the preliminary steps of something before progressing to the next stage. I know you're eager to code your whole website, but you need to crawl before you walk—try doing this test code first. The stages of the game force you to crawl before you walk because they only get progressively harder. There's a reason your music teacher is making you learn "Hot Cross Buns" and not "Stairway to Heaven"—you have to learn to crawl before you walk.

you have to (learn to) crawl before you (can) walk

One must understand or become adept at the basic skills, techniques, or details of something before moving on to that which requires greater skill or experience. I see it all the time—people who want to become hotshot doctors but don't want to put in all the hard work the field demands. You have to learn to crawl before you can walk. While the results of the drug trial are encouraging, there are still a huge number of tests we have to perform—you have to crawl before you walk.