I have a little trick for this because I never remembered the actual steps to take. What you want to do is take their distance from the reflexion axis and double it. For example:
Reflects across y=-1
R: (2,0) Distance from y=-1 is -1
double it is -2
R: (2,-2)
S: (1,3) Distance from y=-1 is -4
double it is -8
S (1,-5)
T: (4,2) Distance from y=-1 is -3
double it is -6
T: (4,-4)
If you can figure out how I did this than good job, this should be easier for any other translation problems.