Ensure that all your adult students know the alphabet by sight, the sounds of the letters, and how to read and spell. This first basic step is crucial to learning how to write: If they don't know the basics, writing words will be meaningless.
Teaching handwriting to adults should begin with the basics of letter formation. Making vertical lines, top to bottom, and writing circular letters in a clockwise fashion are key components to making handwriting flow.
Repeat, repeat repeat. Repetition is key -- just as it is for children in kindergarten. Have your students practice writing capital letters and lower-case letters over and over, printing them on traditional lined paper. Have your adult students print block-type letters until it is they have it mastered.
Teach your adult students how to form cursive letters. This will take much more practice, because while cursive letters flow, they connect in different ways.
Have your students practice writing words using cursive. Have them practice staying in the lines and using proper technique. At this point, a longer writing assignment can be beneficial, such as copying a paragraph or half a page from a book.