Noise level estimation using Hildebrand & Sekhon method

I have some data from a radar to process in order to find the moments and in turn the wind speeds. I have no prior knowledge in signal processing so it’s giving me a hard time to estimate the noise level from this spectrum.

A sample power spectra for a range bin (height level) is depicted in the following image:

I have already done windowing (another question might be needed whether I did it correctly or not) using a hamming window (by the scipy.signal.hamming function) and FFT is applied (using scipy.fft.fft function). Now I want to estimate the noise level using the Hildebrand & Sekhon method, and this is where I have got stuck.
Understanding it without some backgroud in signal processesing has proven to be quite difficult for me to implement the noise level estimation.

Could anybody explain how can I estimate the noise level in this spectrum using the Hildebrand & Sekhon method.

Notes:
There’s 1024 NFFT points in the spectrum.