A leather punch is quicker and easier, but I try not to buy odd tools I'll likely never use again, so I used a drill and small bit. Tied with some outdoorsy cord from local BP/climbing shop.

I made the mistake of using the Brooks apron embossing as a reference point for spacing the row of holes. Turns out the Brooks stamp was ~1cm off from left to right side, which coincidentally was the spacing for my holes, so I remedied the problem with 2 more holes. Be sure to measure the Brooks stamp location to make sure it's an accurate point of reference before you start making holes. Evidently they simply "eyeball" this step of the manufacturing process without any measurement.

The holes should be located rearwards of the embossing to control saddle spread - up front they have little effect. Tying does control apron spread and reduces saddle sag somewhat too, but it will still sag some even after tying.