Ok here goes,
Firstly undo the buckles, Then extended the straps as far as possible. This will make life easier later on. Then clip them together in a loop. They will be clipped with the buckle facing the bag. This is how will be around the tyre. Undo them again
Make a loop like this. I have sat the basketball in there to show how it should be. Where the ball is sitting is where the tyre and rim are.
Place the buckle through the rim like the pic above.
This is where things get a little tricky. Place to top section over the tyre. You may need to loosen off your spare depending on how much room you have for you hands before placing the bag on. It can be very fiddly the first time but once you have the buckles in the correct postion you can leave them.
I have the strap loose in these pics.
Once the clips are connected I use the tensioner buckle to hold it tight against the tyre. It will loosen off from time to time but its easy to fix. After the Kays I have with mine swinging off the back it has never fallen off!
You will have excess strap left after you tension the bag tight. i just tuck this back through to stop it waving about.
Hope this helps a little
Cheers
Rich