There are different hydraulic brake systems and they vary slightly in their bleeding procedure but the principle is broadly the same.
I am bleeding some Hope C2s on a friend's gnarly bike.
They are old but beautifully engineered and are closed system hydraulic brakes.
There are kits to make things easier but this is the old school way using a simple hose.
Step 1: Tools and consumables and naming parts.
You will need an 8mm spanner ( preferably ring ended).
A 26 mm spanner or an adjustable.
Some clean rags.
A small length of tubing or a bleed kit.
The correct brake fluid. ( Some use DOT some use mineral )