A simple frame of cardboard
The idea of making a very simple frame of cardboard is from Franco Rios.
Please have a look at the nice cardboard frame he is using in his No Cut Sprang Bag.
My own cardboard frame is just a piece of cardboard with some cuts to the sides, as you will see in the pictures of the small starting project below.

Working with 2 colors as a help in interlinked sprang
I have noticed that working in 2 colors can be a great help to learn interlinked sprang. Herefore you have to alternate the 2 colors:
To be able to pass your yarn over and under the top and bottom cords, I wind my yarns at fish shaped yarn bobbins (see picture).

When you set up your sprang, be aware of the position of your top and bottom cords. Thet have to stay horizontal. You may have to adjust them now and then, while winding your yarns around them. 

You start with color A at the left bottom (attach it with a slip knot to the bottom cord), you go over the top cord from front to back and under the bottom cord from back to front - leave it and attach color B. With color B you also go over the top cord from front to back and under the bottom cord from back to front Now you pick up color A again. The thread goes under the thread of color B, over the top cord from front to back and under the bottom cord from back to front. Pick up color B and continue this way until you have as much as threads as you want to have for your start. It is often best to have an even number of threads. I started my small sample with 24 rounds of threads).
Then you prepare the fields in the normal way, before you start your sprang (see step 3-5 in this link).
As you can see at the picture, I use a stitch holder instead of a safetycord (in such a small sample). 

cardboard frame for sprang

Now.... in every plate row you will interlink a front thread and a back thread of a different color and in every overplait row you will interlink a front thread and a back thread of the same color.

2 colors sprang

Patchwork sprang
After you have finished the sprang in the normal way (if you do not know how, go to the page with the general information), you can add a new section to it with a piece of cardboard and a crochet needle, and put your sprang again at the cardboard frame. Thus you can make more length.
Sew together different of these small pieces of sprang, and add some crochet, as you wish.

patchwork sprang