Usually its based on symbol patterns in the letter. If they can decode one or two common phrases the rest kinda falls into place over time.

Things they look for are double consonants or where two symbols are paired together.

i.e if you have a letter where the words "Space and "Race" are used then in theory the "-ace" portion of both words should use the same symbols. So plug that in everywhere you see the corresponding symbols, then work fill in as many of those as possible using other repeating patterns and guess and check.