OK, the 1204 looks to be set up exactly the same as the 24 channel version. You need to hook your computer speakers to the main outs on the mixer. They're in the upper right of the unit. There should be a pair of RCA jacks labelled main out and a pair of 1/4" jacks labelled main out. You should be able to buy an adaptor for really cheap that has a stereo 1/8" female STEREO on one side and a pair of RCA jacks on the other. Get one of these and a dual RCA cable. Plug your computer speakers into the 1/8" female side of the adapter. Then plug the rca cables into the rca side of the adapter then plug the other end of rca cable pair into the red and white main out rca jacks on the mixer. Now the Main Output will go out those two jacks, down the two rca cables, into the adapter, where they'll get converted to 1/8" stereo then on to you computer speakers. So anything coming out of the mains from your mixer will end up in your speakers.
On your mixer, in the middle of the right side you should see a white knob labelled Phones/Ctrl Room. Immediately above this knob there's a red button labelled "2TR/USB to Main". When this button is in, the converted sound from your computer's USB port coming into the mixer will be routed to the mains. In other words you will hear whatever the computer is sending out in the speakers. So if you want to play back your mix from the computer and hear it in speakers, push this button in.
Just to the left of the white knob you will see three buttons with a bracket going to an arrow that points to the knob. These buttons control what signal is sent to the headphone jack and the control room output jacks. Unless you have a control room, just think of this knob as controlling the headphone jack. Plug your headphones into the 1/4" headphone jack (you'll need an adapter if your headphones/earbuds are 1/8"). The three buttons to the left of the white knob determine which signal is now going to show up in you headphones. The white knob will make the headphones louder or softer.
Once you have recorded a track and you want to record another one separately so that you can adjust the mix between them later, You need to be able to hear the first one so that you can play along with it and keep everything in synch. For example suppose for the first track you record your basic rhythm guitar part. Now you want to record your vocal. Clearly you need to be able to hear the guitar part so that you can sing along with it. So you put the headphones on and press the top button of the three. It is red also and says "2Tr/USB". Now the sound coming from the computer (the guitar part) is going to show up in your headphones so that you can sing along with it. You only want the vocal going back to the computer for recording though, so you want the "2Tr/USB to Main" button switched out. This way the sound from the computer will ONLY go to the headphones. All that will be left in the main, which goes to the computer for recording is the vocal. Perfect.
Make sense? Main outs to your speakers with an adapter to get from dual RCA to 1/8" stereo. "2TR/USB to Main" (Above the white knowb) should be "In" when you want to play back your mix to listen to it on the speakers, and "out" when you want to record. "2Tr/USB" to the left of the white knob needs to be in when you want to record, so you can hear the mix in the headphones. Headphones get plugged into the headphone jack (with and adapter if you headphones are 1/8"
I think that's about it....
J
PS - Here's a link to the adapter you need to get your speakers hooked up to the xenyx...
http://www.google.com/products/cata...log_result&ct=image&resnum=5&ved=0CCgQ8gIwBA#
You also need a standard dual rca cable.
And here's a link to the adapter you need to hook up earbuds or other 1/8" headphones to the phones jack on the xenyx...
http://www.amazon.com/ER-Stereo-Headphone-Adapter/dp/accessories/B000F2L14M