Best info site/video for indexing gears

[booge]

In short, I'm awful at tinkering with my bike. What I think will be simple ends up with a lot of swearing and an inevitable trip to the bike shop.

I've just installed a new rear cassette on my MTB. Ratios are different so shifting is now off.
It's a Shimano SLX 12spd rear derailleur.
I'm hoping to sort this out without paying a bike shop for something which I know will be do-able.
A lot of YouTube vids are confusing or don't explain it very well.
Where/what do I look at for well explained instructions on indexing the gears.
PS: the cassette is the correct one for the group set

[OnTrackZeD]

The Park Tool video's are good and 10/11 speed video's use the same principles so don't just watch 12 speed videos.

[Duck!]

12-sp. is pretty fiddly due to the very close sprocket spacing. If you've changed cassette size you'll need to adjust the clearance under the big sprocket using the b-tension screw on the back of the mount/main pivot. Shimano 12-sp. derailleurs have an alignment line across to top of the inner cage plate; you'll need to work from the non-drive side of the bike to see it. Shift to the big sprocket and adjust the b-screw so the tips of the tallest teeth align with the line.

From there it's exactly the same process as tuning any other bike, you just need to be finer in your tweaks.

Another thing to look closely at is the fit of the two smallest sprockets, because they interlock with the one underneath and not with the hub. The interlocking splines on the sprockets are very fine and easy to misalign. If they're out of whack they won't sit straight and will chuck the tuning out. There is a small dot stamped in each of the sprockets to line them up properly.