Disclaimer: don't buy rocket motors you aren't allowed to own or fly, don't use my guide alone (use the assembly diagrams available on the aerotech website!!!!), your mileage may vary, etc.. However, I have directly supervised probably almost 100 motors using this technique with zero failures, which is a pretty good success rate.
DO NOT, UNDER ANY CIRCUMSTANCES, TOUCH ANY EXPOSED PROPELLANT FACE. GRIP THE DELAY ELEMENT AND PROPELLANT GRAINS BY THEIR SIDES (COVERED IN PLASTIC ON THE DELAY ELEMENT AND PAPER ON THE PROPELLANT GRAINS), NOT THE ENDS. Most of the time nothing bad will happen, but the Mojave green propellant is toxic, and the delay element can fail to burn if you get oils from your hand on it, leading to a lakestake.
Step 1: Delay Stack Assembly
DO NOT TOUCH THE ENDS OF THE DELAY GRAIN.
The Delay insulator needs to be de-burred, the step referred to in the instructions as chamfering. This is often unnecessary, but is easy to do with a bit of sandpaper-the idea is to remove the rough edge on the ID of the paper tube.
If you are using a delay adjustment tool to change the delay time, now is the time to do it.
After deburring and delay adjustment, assemble the three elements as shown in the image. If you drilled into the delay to shorten it, the hole should face the delay spacer, not the protruding side.
Step 2: Delay O-Ring
Step 3: Assembly of Forward Closure
Grease the inside of the forward closure. Use a finger to smear grease over the entire interior surface. You will inevitably plug the touch-hole (if this is an open closure) with grease, as shown in the second image; use a small screwdriver or the like to clean it out, so it looks like the third picture.
Take the neoprene washer and lay it flat in the bottom of the delay well, as shown in the 4th image:
Now, take the assembly from the previous step and force it into the forward closure as shown in image 5. The aft delay spacer, hollow space, should face backward, toward the propellant, again as shown. It will take some force to get the oring to compress properly for the stack to fit in place; this is most easily done by placing the stack down on the table oring up and then using your palm (while standing) to force the closure over the stack.
Finally, take the forward main oring, lube it similar to the delay oring, and then stretch it over the forward closure as shown in the 6th image. If it is loose, it is probably the wrong oring; consult the assembly diagram and the oring identification chart.
Set this assembly aside for the time being.
Step 4: Front End Assembly and Liner Insertion
NOTE: if your motor has a black hard phenolic liner, it should not be greased.
Place one of the black hard washers (insulator discs) on top of the liner, as shown in the first image. pushing on the washer, slide the liner the rest of the way in, so that the threads inside the case are exposed. Then, take the assembled forward closure (including the last oring) from the previous step, and screw the forward closure onto the case-make sure not to drop the oring, though that shouldn't be a problem. The insulator should sit up against the oring on the forward closure. The excess grease from the liner insertion should protect the threads.
The far end of the case should look like the second image, shown next to the stacked propellant grains we will use in the next step.