To use military camouflage netting the best thing is using the actual pole and spreaders, if you can't get them they I would look at what you could do to make a pole and spreaders. if you are not going to be too tall, a 2"x4"x8' would work with a light flexible board nailed in a manner to give you three arms, to hold in place. Do this times the amount of Camo you have. a set consist of one diamond and one hex piece. Generally you need two sets to put up anything of size to cover a vehicle. One set won't really cover a Humvee to give you a size relationship. Piecing them together is simple if the clips haven't broken off. There is a cord with a bunch of plastic pins on them. You match the male and female connectors together and slide a clip the through and do that every foot or so till you reach the end. For two sets you would need 4 connectors.
Before you buy, inspect, inspect, inspect. That stuff is either low usage or worn the snot out of it. Open it up and check to make sure you have a Hex, a Diamond, a repair kit, and the connector ropes. a Pole bag, should have a decent amount of poles (poles may be either Aluminum or fiber (I am not going to say carbon fiber, but fiber), Spreaders: Spreaders have three arms, open them up and see if the a) the spreader will loosen up, b) the spreader will tighten to the point where it will hold the spreaders where you want them (should look like a Mercedes three point star). Poles: Poles should not be oval shaped, if they are anything other than perfectly round they won't work. See if they connect to each other. if they don't walk away. Repair kit, may have a few small pieces of camo in them, if you use them match the pattern to the correct side. Many people don't know that Mil issue camo has two sides spring/summer and fall/winter. Often a repair can be spotted by the wrong side of a repair sticking out.
It is not uncommon to see dry rot, mold, rips, tears, missing sections, huge holes, all in the same section of camo, so be selective when looking the stuff over.
Lastly, after staking everything to the ground or tying it off to something else, then open up your doorway along one of the connected seems. Don't wear shirts with buttons when going in or out, never have your muzzle up in and around camo, grabs both you and your shirt, and or rifle every damn time. Lastly exposed fishing lures, forget just don't.
Layout, when laying camo out, we always had it rolled and on top, so we rolled it off the long ends of a vehicle first, then unrolled the sides, when rolling up do the opposite. Poles go in when the sides are unrolled. If using Mil spreaders, you may find it easier to use a one section one spreader and then push out and put the second pole section into the first, then the third even a fourth, depending on how much camo is there. As a rule we always wanted 6-12 inches on the ground, but that was so satellite imagery would have a harder time, for you not so much of an issue.
I hoped that help, and yes I have put it up...a lot more than I care to admit.
Sorry it was kind of out of order at the end.
For a 12 x 24 building I am thinking a minimum of 3 sets, most likely 4 would be ideal.
Here is a link with an image of them being together and how to cover a vehicle.
https://www.globalsecurity.org/military/library/policy/army/fm/20-3/appc.htm