The best time I found was in the evening, after work. Have a semi-full bladder (not to the point like you feel you're going to explode) and get plenty of aloe vera gel on your tummy. In the early days (week 8-9) baby was low down near my pubic bone. Starting around week 10 baby started to move around and since then, I can find baby about midway from the pubic bone to my belly button.
Start at one side of your tummy and move S-L-O-W-L-Y. At each spot you stop at, angle the wand in all directions and pause. Then move on. Its a slow, tedious process until you get the hang of it and have a general idea where baby likes to hang out. The first time I found the HB, it took me 45 mins. of searching. The next day, it took me 30 mins.! Now I find it in about 15 seconds or less. You need to listen carefully now though, since baby is older he/she can move away from the wand and make it hard to get a good reading. In the early days, baby wouldn't move much so it was easy to just put the want on him/her and just listen forever.
Good luck!
Here is a recording of what mine sounds like. The fast beat is baby, slow beat is me and the whoosing/wind howling sound is the placenta.
https://s15.photobucket.com/user/mara16jade/media/More Pictures/20131218_173955_zpsnvofe2wv.mp4.html
You can go on youtube and see what other people's dopplers sound like too. Good luck!