We had a previous run with this technique/theme in May of 2016. Beau created examples at that time and included the description below as she hosted that group art exchange. You can go to the SEARCH bar on the right side of our home page and type in "Drips & Splatters" to see what participants made.
Since some of us are soon going to be in the rainy/melting season (Bye-Bye Snow!!!), this seemed an appropriate time to have a re-run of the fun!
For those who are "exuberant" with their medium-flinging, may I recommend placing your blank post card stock in an open box to keep from accidentally splatter-painting/inking your surroundings. (Thin rubber gloves might also be useful... And wearing a smock or apron?)
Masking will allow adding details or focus later without spatters/drips on them.
Another idea: Work with a larger piece of cardstock, drip or splatter to your heart's content, cut down into as many postcard-sized pieces as you like, then finish off each one.
Materials may include watercolors, inks, acrylic paints (possibly watered down), food colorings, dyes, etc. Some inks work better on coated paper like photo paper. (Spray inks might work as well...)
You can add details (collage, rubber stamped images, stickers, etc.) to your dried splatter/dripped backgrounds and- Voila!