The Brompton G Line Rohloff conversion is pretty simple to fit – here’s how to do it:
Remove the original wheel, gear shifter and cable. Fit the tyre and tube from the original wheel to the new Rohloff wheel, and also fit the brake rotor using the 4 bolts that are in the Rohloff kit. 
Remove the left and right dropouts and tensioner from the frame, and unbolt the brake caliper. 
Fit the new left dropout. using the bolts from the original left dropout. 
Fit the old left dropout on the inside right, using the new long bolts supplied in the kit, and fit the new tensioner on the outside of the dropout. 
Fit the skewer to the Rohloff wheel – the skewer goes through from the left side. 
The skewer’s nut goes on the right side – just thread it on a bit, not too tight. 
Slide the wheel in, making sure that the tab on the Rohloff goes into the slot of the dropout. 
Make sure the skewer nut goes inside on the right, and tighten the skewer. Now fit the chain (shorten if needed), Rohloff shifter and cables, and refit the brake using the +20 adapter in the kit.