You'll need to figure out the diametral pitch of the gear. This can be ascertained by a measurement of the pitch of the worm thread. Diametral pitches are related to diameters of gears, so that means that the tooth interval repeats in Pi inches. If your worm thread was that long, you would probably get a whole number of thread "teeth" in exactly 3.141". You can estimate with a shorter measurement spanning fewer threads. But it won't work out to even inches in most cases, although I have seen one worm gear from an ancient box that was cut on circular pitch and it had 1/2" circular pitch and the worm thread was 2 tpi.
You can only rough the teeth on the wormwheel with an ordinary milling cutter. The true helical form is generated with a hob. For a power transmission gear, you may not be as fussy, but it takes a lot of trial cuts and hacking to do it with a regular mill minus the hob. I have done this with manual mill in the past.
I used to grind up a simple flycutter resembling an Acme thread tool but ground according to the width on the gear charts I used.
I would swivel the table of the mill to the helix angle of the worm. Take a cut in the middle as for a spur gear. Go all around roughing the teeth this way. Now, offset the cutter by one thread pitch endwise, and raise the knee until the tool is at the same depth as it was before. This simulates the worm rolling out of engagement with the wheel.
You can interpolate this way as many times as you like, but it will still not generate that true helical twist of the tooth. But, then you can blue the gear and roll the worm around it as if it were a spur gear, to rub off the blue and see the contact spots. Then, swivel the universal mill table a bit one way, then the other, and take a light shaving off the high spot.
What you end up with is a facetted tooth. Surprisingly you can make it fit eventually, although you need to know the true center distance between the worm and the wheel so that you can measure across and tell when its fitting deep enough.
That is a cabbage method. The better way is to finish with a hob but you won't want to buy one of those.
You can also use a lathe and a boring bar, or a cnc mill with a rigid tapping cycle to move the bar, and get a true helical path for the tool. However, this is still incomplete generation because the tool tip only occupies one of a large number of possible positions that a multi-tooth hob occupies at any one instant. That is why you have to take 3 or 4 cuts, turning the wheel a 1/4 or 1/2 turn and rerunning the tool in the new position.
Complex? You bet. Makes spending $100 at Browning a bargain, but you won't learn anything from buying a gear.
Have fun