Hiya Nichola
Put the chain into the middle gear on the front.
Now get the chain onto the smallest cog on the rear, use the H adjusting screw to line the rear mech jocky wheels up with the smallest cog, so the chain is running in a straight line.
Get the chain onto the biggest gear on the back and repeat above using the L screw.
The stops are now set, next slacken the cable tension off to the rear mech. Then start to reapply tension while trying to change gear (up and down) you might find that the difference between smooth shifting and not is only takes half a turn or so.
Hope that helps, if not then we’re running one day MTB maintenance courses this year along with everything else we do http://www.forestfreeride.co.uk
Cheers Stuart