Well, I use whatever pump sprayer is convenient. Currently I have one that actually says Roundup on it. I often have it set so the spray pattern is fairly narrow, and for cracks, keep it close to the ground.

I have plastic sprayers fron 2 liter size to 15 gal. that sits in the trailer behind the lawn mower which powers the pump, all work for Roundup.

The spray nozzle itself is important. You don't want the kind that makes a fine mist, because Glyphosate will kill pretty much any plant, broad leaved or narrow leaved. So use something that has minimal spray drift. You might consider removing the spray nozzle altogether, and just dribble the stuff into the cracks between flagstones or whatever. Avoid spray drift with glyphosate.

Incidentally, Glyphosate does not work as a pre-emergent weed killer. It only works on growing plants.