Start with contents
Measure the item and add 0.25 to 0.5 inches in each direction. A box that is too exact is frustrating to fill and easy to crush.
Score before folding
Score every fold line with a bone folder, dull butter knife, or scoring board. Scoring matters more as cardstock gets heavier.
Tabs and glue
Use tabs around 0.35 inches wide for tiny boxes and 0.5 inches for larger boxes. Tacky glue gives time to align corners, while double-sided tape is faster for batches.
Paper weight
Use 80 lb cover for most favor boxes. Use 100 lb cover for boxes that hold heavier treats, but test the printer and folds first.