Below are the mediums that I used...
Attach the template to the cardstock with a few pieces of washi tape. This step is important since you don't want the template to shift as you are applying the medium.
Apply the medium to the paper using a palette knife. I tried both gesso and flexible modeling paste. While the end result is very similar, I felt that I had more control with the modeling paste. It is a little easier to spread since it is a bit thicker. Both contain a fair amount of water though, so they will cause the paper to warp somewhat as you are working.
Once you are done applying the medium, carefully remove the tape off and lift the template up to leave you with a crisp image.
I decided to add more medium to the opposite corner using the same template.
At this point you will need to be patient. It takes a few hours for the medium to dry completely. You should wash the template immediately after using it. Keeping your template clean will ensure that you continue to get crisp results! Templates can be cleaned with warm, soapy water or with baby wipes. Both methods work well. Just make sure you clean them before the medium begins to dry!
The end result is some lovely texture added to the cardstock.