Updated due to design error, sorry

You'll need 4 x 25mm straps with buckles ( I used Decathlon Backpack straps, £2.99 for a pack of 2) and 6 X 3 bar slide buckles, only 4 shown (ebay £1.99 or thereabouts).

This first bit is where the design error was so has now been changed, all the rest is the same.
Put both straps through a 3 bar slide buckle and slide it up to the strap buckles, note the strap buckles should face the opposite direction to each other.

Feed a 3 bar slide buckle onto the straps, one strap through each side, it must be free to slide up and down.

Feed one strap through its own buckle to form a loop, this will take the bar bag.

In the next picture the white thing represents the handlebar and the red thing the fork crown. Feed a 3 bar slide buckle onto the other strap near the fork crown in the manner it is intended to be used.

Take the strap around the fork crown and feed it back through the 3 bar slide buckle then tighten it up against the fork crown (this stops the harness rotating around the bar and fork crown).

Lastly, feed the strap through its buckle and tighten it up tight. The loop to the left is around the bar and fork crown, the loop to the right takes your bar bag. (Obviously you need to make two).

It should look like this when mounted.

