BMW R1100GS & R1150GS
Tankbag Online Instructions
Use these instructions for the
BMW R1150R & RT
Exception is the under the seat strap. see last photo
  
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 Front two straps attach, one on the left and one on the right, to the bar that the gas tank slids onto.

Left attachment point shown at left using a zip tie poked through the webbing making the task easier.
  
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Left attachment point shown at left.

String webbing through loop at the end of the webbing.

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 Snug down
  
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Right attachment point shown at left using a zip tie poked through the webbing making the task easier.
Right attachment point shown at left.

Loop webbing around bar
Right attachment point shown at left.

String webbing through loop at the end of the webbing.
The third strap actually attaches to the seat.
The seat adjuster bar, as shown at left.
On the
R1150R & RT
The third strap actually attaches to the seat adjuster on the frame not the seat, as shown at left.