This one is very tricky.
I am quite flexible so it is possible for me but may not be for others. What I do is I sit in somewhere private (ideally a bathroom) and take my clothes off. I then pull my legs up to my now erect penis and begin to lick it. After a while I can then begin to actually but the head in my mouth and I start to lick the neck (that bumpy reddish part near the head). I tend to cum in my mouth, it's a lot of fun. See if you can manage it.
Love the site.