Question about the Sonotrax B pocket fetal doppler? If you have ever used it do you have any tips?

I recently purchased the sonotrax b fetal doppler from amazon.com because someone suggested it. They said they heard the heartbeat at 12 weeks using it. The purchase info said I could hear a heartbeat between 10-11 weeks. I am 13 weeks 1 day today with twins and I tried to use it. The display kept coming up with numbers but I couldn't hear a heartbeat. Am I doing something wrong? I was standing while using it so should I try laying down? I am a thin person so it shouldn't be too hard to pick it up. Any tips would be great, thanks!