The Wet Dry Try App, provides explicit instruction for letter formation based on the Handwriting Without Tears (HWT) curriculum. Below is a link to research review articles related to the HWT curriculum :
Additional supporting research articles:
Case-Smith, J., Holland, T., Lane, A., & White, S. (2012). Effect of a coteaching handwriting program for first graders: One-group pretest-postest design. American Journal of Occupational Therapy, 66, 396-405. http://dx.doi.org/10.5014/ajot.2012.004333
Denton, P.L., Cope, S. & Moser, C. (2006). The effects of sensorimotor-based intervention versus therapeutic practice on improving handwriting performance in 6- to11-year old children. American Journal of Occupational Therapy, 60, 16-27. http:dx.doi.org/10.5014/ajot.60.1.16
Dowrick, P. W. (1991). Practical Guide to Using Video in the Behavioral Sciences. New York: John Wiley & Sons, Inc.
Weintraub, N., Yinon, M, Hirsch, I.B. & Parush, S. (2009). Effectiveness of sensorimotor and task-oriented handwriting intervention in elementary school-aged students with handwriting difficulties. OTJR: Occupation, Participation and Health, 29, 125-134.
Young, S., Laxman, K. (2014). Teacher perspectives on the use of mobile devices to improve learner engagement and motivation. International Journal of Mobile Learning and Organisation. 8, 112-129.
What other research supports methods of handwriting instruction?