As mentioned the manual is the best place to to start. I definitely agree that many manuals are fairly poorly written.
Many Youtube videos are very helpful actually, with the first one that pops up usually being spot on.
If you need help from there, the marker specific forums here usually have stickies on maintenance. And helpful members to answer any questions not covered.
While many markers can be similar, they will all have their little quirks to make them different. There is NO end all way to maintain paintball markers. You have to look for a specific guide for a specific gun.