I’ve used a drill press to great effect, to complete my 80% lowers. Most jigs require you to use either a handheld router or a drill press/mill. You can also use a handheld router, but this requires more time and is less accurate/controllable.
Obviously, a mill is preferred but a drill press will work just fine. You first have to drill pilot holes through the top of the receiver, where the fire control group cavity will be made. Drilling these pilot holes eliminates most of the raw aluminum inside.
Some thinner aluminum strips and pockets will remain, but these can be easily cut out using and end mill bit and drill press. You just need to take your time and ensure you’re not applying excessive pressure to the drill press or chuck (they’re not intended to have large amounts of lateral load placed on the chuck/bearings).
Once the FCG is milled out, all you need to do is drill the hammer, trigger, and safety selector lever holes.
IMHO, drill the holes for the trigger and hammer pins and the safety/selector first, and only about 3/8 of an inch deep, from both sides. That way, when you drill out the FCG well, it'll clear out their holes and avoid chips and burrs where their holes come out.
It'll work either way. But drilling the pin and selector holes first is easier, neater and more elegant.