Gelatin is very easy.
You just mix twice the amount of unflavored gelatin in water.
you can gel it in a baking pan but then you have to use paper smaller than the pan.
if you want to use bigger paper or to shape the gelatin you can use regular modeling clay (the kind kids use that never dries) and shape a dam for the gelatin. This works well on plexiglass. Be sure to test it with water and seal any holes. Remove the clay after the gelatin gels. Keep the gelatin stuck to the plexi so that it doesn't move or stick to your paper.
Then you roll or paint ink onto it.
You can put ink on then place an object down to make an impression (don't have to press, water will pool just like with watercolors).
You can use stencils.
you can cut grooves into the gelatin to make "white" areas with no ink. This does hasten the demise of the gelatin though.
Speedball inexpensive water based inks work the best of those I have tried, but you can try other inks or paints too.
You have to work fast if your ink or paint dries quickly (as with speedball). It helps to use a retarder to slow drying time.
Place paper on top and use the palm of your hand to press.
Dry paper meant for silk screen printing works well but you do have to work fast. Thin Japanese papers also work well. I have used flat not textured watercolor cardstock to make cards too.
You can reuse the gelatin over and over for days, just put in the fridge after using. You will know when enough is enough, it dries out in the fridge.
You can print over and over on the same piece of paper even with water based inks.
You can make borders for the gelatin by using a brayer or brush with a dark color and going around the edge.
The following is not the best piece of artwork, but demonstrates a few techniques. I have rolled a background and the border with brayers, and made impressions with various items. The gelatin plate is made with a clay dam, and you can see that the shape is left imperfect (you can cut if you want clean edges, or just don't roll ink to the edge).