You start it as normal by placing the canvas, frame and cardboard together.
After making it sure it all is on the right place like you want; turn up the work.
Use SilverTape to fasten the canvas on the cardboard. Remember to pull & tighten at every point. If you ask help for this, working with SilverTape is much easier. Start with the longer edges...
... and go through each corner / edge.
At the end secure & hide it all by SilverTape.
If you have background cardboard; place it at the end. Now you are finished.