There are good instructions for different hubs, e.g. Hope, on the relevant manufacturers sites but for older designs then a mtb maintenance book (Steve Worland) or a magazine spread (Mountain Bike Rider) may be sufficent.
The only problem is that sometimes they use clean units in the demonstrations.
Used ones look different and can make your hands smell funny so use gloves.
Remove the wheel then use a chain whip and a socket or spanner with a correct tool remove the cassette. You can use an oil filter wrench but they can bend rings unless you are very careful and use padding. This chainwhip was a few euros from Cite d’Europe.
Remove the locknuts and the axle. Check orientation as you remove bits. if you are unsure use that nice digital camera on your phone. Check for wear. (Pitting, damaged threads etc.)
Remove the ball bearings and check for wear in the internal bearing surface. If the ball bearings are old then replace them because they are cheap. You can check them for roundness between two hard surfaces e.g. kitchen worktop and hard drinks coaster. If you are really keen they can be measured using a micrometer or equivalent.
Clean, new grease, replacement bearings, refit axle, refit outer cups and locknuts. Tighten sufficiently and check wheel spins freely without wobble.
Clean the cassette, check the teeth for ‘hooking’ and refit with a socket and a torque wrench ideally. They even print the torque setting on th locking ring of the cassette. Refit the wheel to the bike and you can go wipe the the kitchen worktop.