The only thing that really works for me is when I make it a 25 minute hyper-focussed challenge: Set a timer and make the maximum progress that is theoretically possible in that time. No getting water, no toilet breaks, no looking at the phone. Beats 3 hours of getting a glass of water, toilet breaks, getting hungry, realising I should work out and shower first and finding more reasons to jump up any day - surprisingly. Got to always treat it as if it were a competition.
I'll be honest I don't even know how to study, and I don't know how to write an essay.