The kanji 入 means enter. It looks like a flap door where your cat or your dog can enter. When you want to remember the kanji 入, remember it as your cat trying to enter the flap door!
Also notice that the kanji is slightly different from the kanji human. The kanji human 人has shorter right stroke while the kanji 入 enter has shorter left stroke.
To draw a box you need 4 lines add 2 erasers in the middle and you have the kanji 四 (yon)! In the first version of oracle shell and bone, the character was written just like 一,二,三, where they’re literal representation of the number of the lines. However this later evolved to the combination of 口 (mouth) and 八 (divide) radicals.
The kanji 三(san) still follow the literal pattern of 一 (ichi) and 二(ni). It’s basically 3 lines being stacked on top of each other. To remember this kanji, imagine it as 3 pieces of pencils stacked together.
To remember the kanji 二 (ni), just remember two pencils adjacent to each other. For this kanji, the meaning is still very literal where you literally have 2 stroke of lines to form the kanji 二 (ni).
The kanji 一 (ichi) is very easy to remember, it looks like a pencil. 1 pencil on the table is 一 (ichi)
The kanji hyaku (百) has the literal meaning written on the kanji, you don’t believe me? Tilt your head 90° and you’ll see it! This is how you should remember the kanji hyaku (百)!