Ok, my tip as a free learning tool is powertab (or if you already have some sort of midi softwarre). You can lay out your bars, in whatever time sigs you like. I'd just start by having a single chord ringing out for the duration of each bar. That way, you can start by simply trying to play the first note of each bar. So start by just having a repeating passage of 7/8 for a few bars, then switch it to something else, keep playing along with it and eventually it will become second nature. I just think the easiest way to learn is to have that chord on the first note of each bar. Play along with bands like Pain of Salvation - some changes, but not overly complex as a starting point.
OK, on to polyrhythms/polymetrics.
Polymetrics are when you have two instruments playing in two different time signatures. For example, you might have the guitar playing a pattern in 3/4 and the drums in 4/4. Now, if you look at this below, count it as one and two and three....the 1's are the first beat of the pattern. Now, after 4 repetitions of the 3/4 pattern and 3 repititions of the 4/4 pattern, they would return to falling on the same beat again.
1 + 2 + 3 + 1 + 2 + 3 + 1 + 2 + 3 + 1 + 2 + 3 +
1 + 2 + 3 + 4 + 1 + 2 + 3 + 4 + 1 + 2 + 3 + 4 +
Polyrhythms however are based on dividing notes. A 3:4 polyrhythm means 3 into 4. You divide 4 beats by 3. These notes will seem to fall off the beat - they do not fall where you would normally expect them to. It works in exactly the same way as a regular triplet.
I copied this from the meshuggah forums - I think it's a good explanation of the difference between the two (copy and paste into address bar or it wont work) If you imagine it a bit like a midi grid.
http://www.imagestation.com/picture/sraid160/pd49f78e7412f10d784535b4fca49ac94/f4ddb288.jpg
If you imagine each of those little boxes in the first example as a quarter note - the red notes are the start of each pattern.
In the second example, the polyrhythm, the top line are still quarter notes, but the bottom notes are 1/3 of a whole note in length.
These are overly simple examples but it gives you the concept.
I'm sorry this wasn't the best explanation - I know it's probably confused you - I always think these are very difficult concepts to explain in text. I'm at work at the moment, but I'll try and do a better explanation with proper examples tonight....or Ron will just come on here and explain it properly.