Cubehead has a really great video with a document on how to learn.
https://drive.google.com/file/d/1JM798XJ1FlpRF-8oGlil8l2-57axbMXn/view
Check out the whole document
Ok, I'll be reviving this now again.
Generated By csTimer on 2021-11-24
avg of 25: 19.67
Time List:
1. 18.82 R2 D R' B' L D2 B R F' U' F2 L2 D R2 L2 U L2 U' R2 U2
2. 19.67 D F R' F2 B' R D L2 D2 F U2 F2 L2 B' U2 R2 B D2 R
3. 18.08 L' R2 F L2 B U2 B2 D2 U2 L2 F' U2 D R' D' R U' B L2 F'...
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JPerm has a very nice video on how to get sub-20, I'd recommend going over it once.
Just one advice : Slow down and look ahead, and work on your F2L, rest of the solve is good. Also practice and drill your PLLs.
I recently got sub-20 and that's what I did.