Best way to improve aim?

Hours and hours of deathmatch. Training your muscle memory. Also, consistency plays a big role so don't switch your sensitivity and DPI, just pick something you are comfortable with and stick to it.

You also need some time to rest, so playing 48 hours of deathmatch in a row doesn't really help. And progress doesn't come overnight.
I would do an hour of DM every day for 1-2 weeks.