1.) Note the value of any thing on the screen.
if it is a numerical code
2.) Enter its numerical value and and hit search.
3.) Decrease or increase the value.
4.) Search again...keep repeating this and eventually get the value and its code.
if is not a numerical value and changes like e.g. life bars and time etc.
2.) enter a fuzzy search...just hit search.
3.) spend some time and change the value.
4.) do a fuzzy search on it again by telling the program that it changed.
Keep doing the above process for some time and you'll get a hang of it soon.
I recommend using MAME's internal cheat searching system for practice.