what shoud i say in a follow-up email to prospective PhD supervisors after I submitted my applications I am applying for Ph.D. programs. I reached out to several potential Ph.D. supervisors last year and had informal Skype interviews with some of them before I submit my applications. It's been a month now, and I submitted all my applications. I feel like I should send follow-up emails to those professors. I have no idea what to say, should I ask whether I will have an interview or just say I'm interested in their research groups again. I just want to send a friendly reminder for them to review my application. Please help me, I really have no idea and I don't want to send something stupid.
For those with whom you communicated previously, a follow up is appropriate. But it can be very short and simple. Just say that you have applied and thank them for their help. Add that you remain interested in their research and in working with them in the future. 

You don't need to remind them of anything. The university procedures will take care of that. The "thank you" part is the most important.