Hold the SPACECRAFT BUS with its open side down, above the flat rectangular end of the PM TRUSS. Notice the slots in each of the four corners of the PM TRUSS. Also notice the crosshatched markings on the lower tabs of the SPACECRAFT BUS. Position the SPACECRAFT BUS so that the tab marked PEP is over the triangular area marked PEP on the PM TRUSS.
With the SPACECRAFT BUS in one hand, PM TRUSS in the other, slip the crosshatched shelves of the SPACECRAFT BUS into the corner-slots of the PM TRUSS, at the PEP end.
Squeeze the PM TRUSS so the three remaining sides flex toward the middle. This will allow you to work the PM TRUSS up inside the bottom of the bus, putting the rest of the crosshatched shelves into corner slots of the PM TRUSS. Now straighten out anything which was deformed by squeezing.
Align the PM TRUSS so that the triangular PEP is parallel with the face of the SPACECRAFT BUS segment whose tab is labeled PEP. Square up and center the PM TRUSS within the SPACECRAFT BUS. You might sparingly apply a few drops of glue to hold it in place, but DO NOT allow any glue to obscure the corners of the PM TRUSS. Tabs will need to go there later.
From sheet B, release the REM (Rocket Engine Module) LOWER SUPPORT. Look for four small notches in the circular ring, each one between a pair of triangular struts. Bend the four tiny black tabs at the apexes of the triangular PM TRUSS struts over about 90 degrees toward the center. Hook these tabs into the four notches of the REM LOWER SUPPORT. You'll have to stretch the triangular trusses outward a bit. Secure with glue, being careful not to get glue in the two holes on each apex.
Image shows the PM Truss and Helium Tank installed (it also shows the REM supports, and REM Lower Support, which are to be added later).