The rules not letting use a letter you know isn't in the target word drastically cuts your choices, your best bet is to use as few unique letters per word as you can, and as uncommon letters as you can, but as all normal words need vowels you'll be forced into only picking a vowel from the target word soon enough. Try to find letters that are in the word as late as possible so you're not locked into having to use them.

Once you are forced to use letters from the target word, avoid placing them in the right place. Sadly there's a rule not letting you use a known letter more than once in a word (once A is known, you can't write ALPHA), and a rule for not letting you put a known letter in a place you know it doesn't go (so after MAMMA, you can't have HALVE because you know there's not an A in second place, nor ALPHA because you know there's not an A in fifth place).

trash smash abash gnash quash clash

    ALMEH
    BEKAH
    BELAH
    DEASH
    OBEAH
    SELAH

e.g. show a word with some green letters and that they have to be in the same spot in the other word. e.g. FUBAR with green B -> __B__

FUBAR with yellow R -> RRRR_ (with gray background and white R to signify "has to be placed somewhere here")

And for the gray letters, I guess.. show them next to the _____ in a trash can?

I tried WEIRD and it rejected it as not a word