I've also used those upholstery pins to do a "temporary" fix on my X300's droopy headliner. They work well.
One tip is that you can make the job at least look symmetrical by sitting in the centre of the rear bench seat, at least until you feel you can't comfortably reach far enough forward towards the front windscreen. I started to do mine seated in the driver's seat and wondered why it wasn't easy to make it symmetrical
. Sitting in the middle of the car lets you work out a plan of where to put them while having an easy overview of the whole headlining, which is easier when aiming for an even placement of the pins.