Surprisingly quick and easy to make!
There are plenty of tutorials out there. For a very quick potted version (and to remind me when I'm looking back wondering how on earth I made it!): You need 1 sheet of A4 card and a scoring board. Score on the longside at 7, 14, 21 & 28cm. Then score on the short side at 7cm and cut it at 15cm. Cut down the shorter folds to make 4 flaps and glue to make a box shape. Using the discarded smaller strip you cut off, cut 2 strips with a central 7cm piece and a scored 1cm border at each side. Put glue on the 1cm borders and use these to adhere the 2 strips inside your box. You then stick lots of embellishments on these strips to make your box look overflowing. Decorate profusely!