It should have come with instructions... Anyways, the assembly is pretty much the same as a standard receiver except for a few points.
Since the receiver is molded plastic, you'll probably need to use and exactor or sharp utility knife to remove and flashing that may bind the parts. Also, lighty spin a drill bit in every hole by hand to remove any burrs or loose plastic that may be hinding in the holes. Be carefull not to actually drill into the holes or widen them.
There is a one inch long roll pin that comes with the receiver. It is used to hold the buffer retainer in place. After inserting the buffer retainer spring and retainer into the receiver at the opening of the buffer tube, you need to hold the retainer down while driving the roll pin in from the side. You'll know what I mean when you see it.
You obviously don't need the pivot/takedown pin detents or springs. Save them, or do like I did and give them to someone who accidentaly shot a takedown detent across the room.
Now, there are two places where you might need to cut springs: the bolt catch spring and selector detent spring. I say might because some LPK spring may differ from others. With my RRA LPK I had to cut 1 and 1/2 rings off the bolt catch spring before my bolt would lock open on an empty mag. The same mags will lock the bolt on my Stag lower without modification to the bolt catch spring. Since the roll pins are much easier to insert into a plastic receiver, I suggest clipping 1/2 a ring off at a time and testing with your upper and every mag you've got untill the bolt locks open reliably.
With the selector spring, drop the spring thru the selector detent hole from the top. The insturctions say to trim the spring so that it is flush with the selector hole. Then you drop in the selector detent. Finally, you insert the selector and use a punch/rod/anything to push the detent into the hole so the selector can get through. Be warned, once you install the selector it will be very difficult if not impossible to remove. Make sure you test fit every part in the receiver before installing it. I hurried through the instructions and missed the part about trimming the selector spring. I also forgot to test the selector in the selector hole. I wound up with a very stiff safety. Fortunately it lossened up after I flipped the selector back and forth while watching TV for an evening.
Little tip: The easiest way to install the bolt catch roll pin is to tape up the jaws of some channel lock pliers or a set of vise grips. Then just squeeze the pliers/grips to push the roll pin in flush. To get the roll pin in just a little further, I used the same pliers and the extra takedown pin detent as a "punch" to push the roll pin just a little farther into the hole.
Hope I didn't miss anything. What color did you get?