Start by understanding what good "standard" ranges look like and use that as a base, then adjust based on villain's frequencies and especially villain's hands when you see them show down. Keep practicing and over time your skill doing this will improve as long as you're putting active thought in at the table.
Yeah, iím going through the same right now too. I memorized all the preflop opening ranges and I use that as a baseline as to what i think they would open with depending on position. From there, iíll try and figure out if they are calling or folding too much, and that helps for me better guess if their starting ranges might be a bit wider or tighter than i originally thought.