Paper, in its way, is a fairly unforgiving medium, particularly on precise angles where two surfaces meet..  (like the tops of the stadiums.) There is no stretching or compressing. A diference of a single degree at the end of an eight inch fold can can mean more than 1/8 inch difference at the end of the fold.

Take your time, score your fold, then examine the score, you don't want the score to vary by more than the width of the line. And use a straightedge to make the fold. If the straightedge has a padded back the lie it on its front so  that the flattest  surface is against the paper for the crease.

Don't use too much glue, it can cause wrinkles, which can change the dimensions.

Mostly it comes down to being careful. You will make mistakes (so do I) so dry fit everything before glueing.

I have been paper modelling since the early '70s, and the simplicity of the World Works line makes it easier than most to put together. And things do fit, there is nothing like the discovery that two castle walls won't meet because of a printing error, a problem I had with some of the French castle models I used to put together - the press changed the dimensions slightly - not a problem with pieces that were lined up the same direction, but if they were rotated to save space on the sheet then it would be pulled along the length of the paper slightly, making it not fit against the supposedly matching walls.

The Auld Grump


*EDIT* If you can find it Post It tape can really help with the dry fitting.