Probably a really basic job - I am trying to get my gears to adjust properly but it has started to do my head in.

I have a 21 spd old hybrid with grip shifts. The problem is I can't seem to get the front set onto the big cog, or if it does go it will crunch down from time to time to the middle. I am not sure if its cos the cable is not going tight enough or if those little hi/lo screws are all wrong. I have been fiddling around but have become frustrated.

The shifts and cogsets were changed a few months back by LBS, but i aint going back there as they did a number of very wrong things to my beloved bike as part of their standard service.

Is there a website with step by step pics anyone knows of?