The sides snap into the top and bottom pieces. You can see in the pictures how that works. The back side of all the pieces should be flat/even when assembled. Probably best to add glue for additional strength.

The outward facing edge should be placed face down on the plate. The connectors on the sides should be flat on the plate for correct orientation. No supports are needed. The connectors on the sides may need a brim if your adhesion is troublesome. The default profile does not have brim set.