Find candidate keys – What are the steps

I have these following functional dependencies I figured out:

DM -> RA RDT -> AM AD -> RM 

I got with a software to calculate what the candidate keys were to this:

{R, D, T} {A, D, T} {M, D, T} 

But I don’t know HOW i should do this manually to figure out this. Not to use the actual software. What the steps are to solving this. First I thought I should do something like this to figure out the candidate keys:

DM+ = DMRA RDT+ = RDTAM AD+ = ADRM 

But from what I understand is that only the RDT+ is giving all the attributes for it to be a candidate key? I am so confused by this. How should I think to pick it out from these functional depedencies?