QoS on Powerconnect 2748

Hello All, 

Im currently having problems with our VOIP Phones in our office. A lot of jitters and dropped packets are happening. We have a Powerconnect 2748 Dell switch which is capable of giving QoS to the voice packets. I was able to do this on the router side already but is having problems with the switch. I cant seem to understand how it works on the switch. I read the manual and still cant get it. Im sorry about this and I hope you guys can help me out. I know the QoS theory but setting it up on the Dell switch seems to be rather challenging. Were do I start? 

I hope to hear from anyone soon. 

Regards, 

 Mike