If you look at the original plug in the main loom there is a Bu/W leading into it. Snip this back from the plug (leave a reasonable amount of wire behind the plug) on one side of the plug, it's up to you which side you snip, loom side or switch side.

Look at the back of the four pin plug that goes into the flasher relay you have:

Bu/W Bu/W
Gr/W Gr/W

Top left (looking from the back of the plug) Bu/W connects to the Bu/W you just snipped leading to the switch. Top right Bu/W connect to the snipped Bu/W leading off into the loom and the center BU/W in the three pin plug. Bottom right Gr/W connects to the Gr/W from the three pin plug that is nearest the groove, the other Gr/W connects to the Gr/W at the other end of the plug.