Nap location:If you have your own office, its easy, just close the door and lay on the floor. I work with an open floor plan nowadays though - but if your company is cool enough to have an open floor plan then they probably also have little phone-booths with chairs you can kick back in. The key to napping upright in a chair is the lean your head back on something (your sweatshirt). Your neck won't support your head once you fall asleep so if it isn't supported you'll wake as soon as you nod off.
Don't nap on your desk. It is best to separate your exact nap location from your exact work location or you might think about napping too much.
Nap length/time:I usually set my alarm for 20 minutes, but find myself waking regularly after about 15 minutes. I wouldn't recommend napping for longer than 20 minutes, but if that works for you then okay!
For me the best time to nap is sometime after lunch, anytime between 1 and 5. I know its naptime when I feel like I can't keep my eyes open and can't stand on my feet. This is easier to feel if you have a standing desk; and if you don't have a standing desk then go and make a standing desk.
Falling asleep / waking up:Don't think about napping or about trying to fall asleep, I usually think about something technical like trying to find a solution for something at work. The key is to keep muscles relaxed, don't move. The truth is it doesn't matter if you actually fall asleep - having a uber relaxing moment is still great!
When you have that moment that you realize you were just asleep - that is when you get up. Don't wait for your alarm, don't lament that it was "only five minutes." Congrats you napped!
Feel free to comment if you have questions!