when i straightened my hair (once i knew what i was doing), i used both a flat iron and a hot comb.
first, i would detangle, moisturize and braid my damp, washed, hair (to stretch it) in several sections (like 10), and let it dry like that.
next, i would remove one dry braid and comb through it again.
then, i would flat iron that section while brushing/ combing it (i would put the brush/comb directly beneath the flat iron while pulling down with both. i would also pressssss the flat iron plates verrrry harrrd together.
finally, i'd go over it with the hot comb, and
press the hair verrrry harrrd again against the flat part at the back of the comb with a rag. this got my hair super straight! i don't think i used very high heat. (with heat there's always damage, but it wasn't noticeable).
hth...
ps: i'm mostly 4a