The more you work with it the easier it gets but I've done Numerology for close to 30 years and still have to use a copy of that chart in order to get things straight (the curse of a failing memory and being dyslexic) But the majority of folks I know learn it with little problem. I believe there are a few "cheats" in one of Ron Martin's books. . . I want to say The Palm Reader's Notebook.
The gist isn't to remember the chart but what 2-3 numbers are listed beneath each number e.g. 4 would have D, M, V under it, the mnemonic is Department of Motor Vehicles. . . 6 = F, O, X. . . bet you know a good memory cue for that already. 9 is the only number with only two letters -- I & R and you know that every 10th letter will be under A.
Trust me, there's a heck of a lot more you'll need to learn. If you like this stuff pick up a copy of my friend T.C. Tahoe's RIDING THE RAINBOW, it's packed with some awesome material.