What is the best way to grow longer hair?

I have 4b (afro) type hair naturally so I relax it to make it much MUCH easier to deal with. I’ve been trying to treat it better and it’s grown a tiny bit, but I’m still not happy. It’s around neck length/shoulder length and has been short for most of my life.

Since I have a different type of hair, I do not wash it every day. I wash it every week or two and only use heat (flat iron on 250 F) twice a month.

What are some tips for me to grow out my hair? My hair has never ever been long before and it’d be nice to finally have some long hair that I can be proud of without having to resort to my clip in extensions.