You look for patterns like common words “the,” “that,” “it’s” etc. This cryptogram had some tricky elements like 1 symbol for 2 letters (TH) and symbols for punctuation. You start replacing every occurrence of 1 symbol with your first guess (like T for rainbow) and go from there.
Call the doctor real quick for that antiviral treatment. Our cases remained mild when we got it right away. It has to be given early in the illness.