(I still don't have properly working camera and that's why I share this a bit archival DIY).
You'll need a plain sketchbook, flat brushes, vicol (or another decoupage glue), pressed flowers or leaves.
Paint the first layer of glue and put your design on it. Paint the second layer and let it dry. When it's transparent, put another layer. Repeat this step as many times as it's possible - until your cover is even. That's all!