IcyVeins is a good resource. If you follow it you’ll do fine. The reason you were last in DPS is because a) you have very low gear and b) you weren’t specced into Beast Cleave so you had no AoE. BM uses Beast Cleave and Kill Cleave to AoE.
BM talent builds here:
Just run the AoE build for all dungeons. Switching builds during dungeons is usually not worth it and you can’t do it anyway in M+. For raids consider swapping to single-target build for single-target bosses.
Rotation guide here:
You have to tick the box next to each talent selected.
The TL;DR of Beast Mastery is keeping Frenzy at 3 stacks via Barbed Shot while keeping Kill Command on cooldown and using Bestial Wrath/Death Chakram when they come up. For AoE you use Multi-Shot every 6 seconds to keep Beast Cleave up.