A Brooks B17 Narrow is very much like a Brooks Swallow, but the Swallow has no side skirts and 2 flaps underneath to prevent it from flaring when seated. But a Swallow is many times the cost of a B17, so cheapskates like me have been butchering their Brooks saddles since their invention for the sleek looks that this offers.
Mark off the skirts where to cut. When the flaps are finished, they are at an angle so that they sit at right angles to the fore-aft centre line. This is difficult to estimate, so simply cut them generously and trim later. Don't make sharp corners where the flaps meet the saddle edge - the curve creates a natural flowing edge after folding.
Cut the marked sections out with a very sharp craft knife. I find it helpful to use a marker pen to mark the cuts, then trace them lightly with the knife before cutting.
Soak the flaps in water overnight. The water saturates the leather and makes it soft and pliable, ready for folding the flaps towards each other. Don't attempt to fold the dry leather as this may break the leather fibres and make the surface look ugly.
After soaking, shape and fold the flaps towards each other, squeeze them further together and clamp in place. Allow to dry completely. This step makes the leather take on the new shape.
After drying, trim the excess material off the flaps. Mark and punch 4 matching holes and lace with cable ties.
With a belt sander, finish the cut edges for the final shape.
Don't burn the leather when finishing.
To limit sag, I put a piece of stiff packing foam in the loop formed by the flaps.
A piece of rubber resting on the seatpost clamp and held in place by the screw, supports the flaps, which is held in shape by the foam.
As installed on my Swift in preparation for the Fleche-Opperman 24h 415km trial.