How to contact the professor again?
I am a graduate student, though I am not getting a research masters. However, what I would do is this:

In most cases, it is a board of admissions counselors made up of multiple professors and maybe some administrative positions that will be making the decision as to whether to accept you or not. So, in that case it means that the decision to admit you into the Research Masters program would likely not lie solely on the one professor that you have listed on the application. So what I would do is contact the main office of the department that heads the research masters program that you have applied to and tell them something like 
"Hello, my name is _____. I would like to check on the status of my application. I have been in contact with professor _____ but I know that he is busy so I wanted to check with you all to make sure that you have everything that you need from me and to see when I can expect a decision to be made pertaining to my acceptance. 

Sometimes calling and speaking to someone directly will get you a lot further than an e-mail, so you might need to try this method. 

I hope this helps!