Firstly: Making an MSE template is a very complicated process. You will need to know something about programming or at least scripting. You will also need to have a way bigger/higher quality image for your backdrop, or it won't look any good when printed (and you may want to remove that scan from your post, as it's not strictly legal 6.6; ).
Now, you can make a style template with your text editor of choice. I recommend looking into an existing "style" file to start with (look for these inside the specific folders inside MSE's /data directory) - this should give you an idea of how the scripting works. It's probably a good idea to design your cards for the
UCS if you plan to share with the community.
From there, the best place to find information about this is the
MSE forums and wiki. Do not ask about D&D cards specifically, though, since WotC's legal team spooked them a while back. Just observe the documentation and ask questions, if you need to, without mentioning D&D.