There is a big difference between knowing the instruction set and being able to program in assembly. I'm not sure where you are starting from; there is a lot of background knowledge to build up before you'll be able to look at a block of assembly code and understand what it's trying to do. In broad strokes:
- Boolean algebra, the foundation of digital logic. How complex "decisions" can be made from simple comparisons between true and false. Learn how to evaluate expressions involving AND, OR, NOT, XOR. It takes practice to learn how to think this way.
- Binary and hexadecimal number representation, and moving between them. Signed and unsigned. What is 0xF in decimal and in binary? How many bits does it take to represent one hex digit? Have you memorized the powers of 2, from at least 2^0 to 2^8?
- Digital logic and arithmetic. What happens numerically when you shift a value left or right? What is a bitmask, why is it used, and how is it often implemented in assembly? What happens to a value when you XOR it with itself? How are addition and subtraction performed? What do the "flags" on the Arithmetic and Logic Unit mean?
- The "game loop" paradigm. Several times per second, the state of many objects is evaluated. The state of each object is represented as a number. That number is compared to other values to determine if the state should change. Does a sprite position need to be updated? Did one object collide with another? Did the player push a button? Does something else need to happen? Over and over, many times per second, creating the illusion of simultaneous action.
- Flow control. Comparison, branching, jumping to a subroutine.
Even when familiar with these things, you must then learn the quirks of the platform you're working with (SMS), and the quirks of the specific ROM you are working with.
It's a lot like an RPG. All you want to do is go to the next town. But first, you have to talk to everyone in the current town until you find the old man who tells you where the cave is, where you can find the mystic axe to slay the giant troll, who gives you the crytal shears for cutting the cursed vine that imprisons the sacred eagle, who, once freed, will call upon his friends to build a bridge across the chasm of despondence so you can walk to the next town. But once you're there, all the enemies kill you easily, so you have to go back across the bridge to the old town and grind for hours until you've leveled up enough to survive in the new town.
I'm not trying to discourage you; just realize there is a lot to learn before you can figure out the exact problem you're trying to solve. These are some good starting points. I'm sure others here could pitch in as well.