This is easier than it sounds. for one...dont try it on vetran.
As Z0rrofox stated, restart if you are detected.
I find the toughest bit is before the junk yard section where there are a few scraped vehicles (helicopter for one) and the terrain is all marsh land. When your scottish friend asks you to either sneak or kill them go prone and slowly go right. Ctp macmillan will go infront of you and also go prone. from there follow him and move slowly