Determining the LF/HF ratio of a signal

Can anyone of you guyz, help me understand the process of determining the PSD of a signal? My goal is to upload a simulink model that determines the lf/hf ratio every one minute. There have been sample codes that I tried understanding, but when I use simulink block, I just get lost. So, I tried downloading some ekg sample signals from physionet, then I opened it in simulink through .mat file. I then attached a hamming block followed by fft since from what I've searched it will then give a PSD. I got the real and imaginary lf/hf ratio based from the scope. From there, I'm all blank. I'd really appreciate it if you guyz could help me. Thank you!