Monster Valves and Mini Monster deflators are installed using a 1/4-inch pipe thread tap.
If you've never used a pipe thread tap before, there are some nuances worth noting.
First, a pipe thread is a tapered thread. Tap a pipe thread hole too far and you won't be able to properly tighten the fitting into place.
Second, you should start the tap into the hole and make some significant progress before turning it back out to clear the chips. Unthread it too soon and it may be a little too easy to cross thread the tap once you start it back in.
Once you've found your ideal depth by tapping a little at a time and then test- threading a fitting into the hole, you should mark your place by wrapping some masking tape around the tap.
You can turn the tap with an open-end wrench or an adjustable wrench as shown here.
Always use cutting fluid or cutting lubricant. I used Edge Cutting Cream.