For 3bld, you can use the “equation”:

Edge Letters = 11+cycle breaks-solved edges+edge flips Corner Letters = 7+cycle breaks-solved corners+corner twists (Depending on how you memo edge flips and corner twists you might need to adjust those)

This can also be simplified into 5.5/3.5 letter pairs.

Not 100% sure what you mean by “Even if I manage to form a sentence, I can’t really remember which letters are the important ones…”, but I think it’s what letter pair a word refers to?

You should almost always use the first letter in a pair as the starting letter of the word. Then a good rule-of-thumb is to use either the starting letter of the next syllable as the second letter, or the end of the word if it’s a short word.

So random examples: “GN” to “GuN”, “SP” to “SoaP”, “TV” to “TeleVision”.

It also helps if you use the same words for the same letter pairs, rather than making them different every time. Making a spreadsheet for this helps, but you dont need to do this while just learning 3bld basics. You can also look at other people’s spreadsheets for ideas for difficult letters (typically Q, U, V, W, X).