Step 1: Plastic bubble
Somewhere along the line you will need a clear plastic bubble to encase your figure. We will work on cardboard backing later.
Where to find
Dollar stores carry cheap action figures and they will work. All we need is the bubble!
While browsing for a suitable cheap figure for it's donor plastic bubble keep in mind what you want to make for accessories for you're figure. As you can see in the pictures the plastic is custom fitted for the Ninja's accessories. Not much else will fit in there. So you might decide to instead go on ebay or shop around for say like a McFarlane action figure. They usually have nice spacious spots for custom accesories.
You can use the Ninja plastic as is or cut out some plastic and then tape your accessories in place. Another idea is "vacuum forming"
Found yer bubble
I'm sticking with my idea of the dollar store bubble.
So you want to get the clear plastic bubble off the cardboard backing without scratching or marring it up. If you can present the action figure in a clear brand new looking plastic bubble it will dramatically add to the wow factor. You like wow faces don't you?
After carefully prying off the plastic bubble I removed the glued on paper with nail polish remover. Some ink residue remained and I'm ok with that. Set this aside for later. Somewhere where it won't be squashed, bent or scratched up.
Eventually you will need to design your own cardboard backing.
I measured this particular action figures cardboard backing and it measured 6 3/4th inch wide by 9 1/2 inches long. You can use this as a template later. Also you can use the slat wall hook as a template to where to cut out on your final cardboard backing product.