But first you have to make sure that the part fits. The sides should slide with a little effort in the slots of the centre section. If not, it might help to ever so slightly touch the tabs with a sanding block to chamfer the corners slightly. Just enough to knock of the edge, not more! Again do use Titebond. This is a big plane, do NOT use superglue!
Watchout: Former 506 must be installed such that the diagonal brace looking in the flight direction, with the fuse upside down as it is now, runs from lower left to upper right. The following formers are glued in such a way the the direction alternates. (Next is lower right to upper left.)
Glue the sides now, and add the 502 and 506 formers before the glue sets. That allows small adjustments, especially at the level of 506 where the sides curve.
Hint: it is easy to use to much glue, and that makes the plane heavy! Remove excess glue after the parts are put together. Use a scrap of wood or something like that to scrape away the excess. And do make sure there are NO gaps in the joints.
Watchout: It is very important that the space in the formers for the corner pieces 527 (8 x 8 mm balsa) stay clean, and that the sides do not get pushed in too far. It must be possible to insert the 527 balsa pieces afterwards!
You can now add the remainder of the formers, (503 to 512, as will as support formers 513 and 525.) To make inserting the ‘feet’ into the centre section easier, use the method described above, i.e. , knock off the sharp edges where the feet go into the centre part.
Use part 618 to set the correct distance between 525 and 511. Part 628 has 2 slots, that are used to align 525 and 511.
Next glue fuse bottom 514 into the sides 501 and former 502. The LG supports 515 (front) and 516 (rear) can now be added. I always sand down the corners of these parts to allow them to fit snugly. Do the same with corner reinforcements 517 at location 511.
The next step, the construction of the lower wing centre piece, needs some additional preparation. You need some cling-film to cover the fuselage.
Assemble (Don’t glue them yet !!) parts 518 to 524 according to the plan and install this assembly at the correct position on the fuselage. If everything was built according to the plan the assembly should slide without problems into the provided space. Only parts 521 are allowed to show a small 1 mm gap over a length of 15-20 mm at the location of the sides. This is acceptable and is caused by the curvature of the sides.
If everything looks ok, than parts 521 and 522 are glued together. Do make a left and a right assembly, and do make sure that both still fit support former 518.