record yourself practicing with a metronome. thats one of the best ways to know how far you're off. sometimes just playing along with a metronome you wouldnt know how much u suck.

if you're talking about odd time signatures. go watch some of mike portnoy's instructional vids. he explains it very well and practical even for guitarist.

if you're talking about 8th notes. start from the very beginning. like learning how to even clap properly to a metronome, move on to tapping your feet properly to learning how to play one note properly before moving on.

since im probably the laziest guitarist around here. i just learn the songs, practice it til it becomes muscle memory and by the time u hit the stage u can just play and have fun without thinking too much (heck, i even daydream sometimes when i'm bored on stage. stuff like where am i going to have supper after all this. lol)