Pesky. That is the only word that aptly describes trying to write with one hand, your non-dominant hand no less, whilst not having anything to firmly hold the paper, greeting card, or whatever in place.
Here is one solution. Cut some of that nifty non-slip pad that I talked about earlier so that it fits the back of a clip board. It does not have to fit perfectly, but does need to be smaller than the clip board. Elmer's glue works just fine to mount the non-slip material to the back of the clip board if you let the glue dry for 24 hours.
Now you are ready to slide your paper under the clip of the clip board (remember to use your four finger and thumb like Tomme-K told you!) and start writing. Any more ideas?