Originally Posted by cxwrench

To start this post should not be in GD, it should be in the proper section which is of course 'bicycle mechanics'.

If you don't know how to adjust the derailleur you should watch a bunch of good YT videos as others have suggested. I would also suggest starting from the very beginning...reset the whole thing. This way you'll have a better understanding of the 'hows and whys'.

1) Put the bike in a work stand.
2) Undo the cable from the derailleur anchor bolt.
3) Screw the barrel adjust at the shifter in all the way, then back out 1 turn.
4) Shift the bike into the largest cog in the back.
5) Make sure the outer derailleur plate is parallel to the chainrings.
6) Adjust the low limit so there is 1-2mm between the chain and the inner cage plate.
7) Attach the cable, pull on it so it's snug.
8) Shift the rear to the small cog.
9) Shift the front to the large chainring (it's a chainring, not a cog).
10) Watch the derailleur as you make the shift, the cage should shift the chain up to the large ring and stop...the high limit controls this movement. If it goes too far the chain will come off the outside of the ring. If it doesn't go far enough it will not make the shift. You want the derailleur to shift up and stop. You don't want it to make the shift then move back at all. This is controlled by the barrel adjuster on the shifter. If the derailleur makes the shift but the chain goes back to the small ring when you stop pushing on the shift lever you have too much tension and need to screw in the barrel adjuster a bit. If the derailleur makes the shift but moves back a little when you stop pushing on the lever you need more tension. You want the same 1-2mm clearance between the chain and the outer plate.