Let's assume for the moment that your sources all offer component video outputs only. Just to be sure we're clear on our terms, "component video" refers to the analog video connection that uses three RCA connectors (typically color-coded as red, blue, and green) to carry the video signal. With that assumption, here's what I'd typically suggest:

Connect a set of component video cables from the DVD player to the 990's "DVD component" input. Also connect the DVD player to the 990 using either a coaxial or optical digital cable (doesn't matter which) - for the same of this discussion, we'll use optical and plug it into the "Optical 1" input on the 990.

Connect a set of component video cables from the HD satellite receiver to the 990's "Video1 component" input. Also connect an optical cable from satellite receiver to the 990's "Optical 2" input.

Connect the CD player's optical output to the 990's "Optical 3" input.

Connect a set of component video cables from the 990's "Monitor Out" component output to the HDTV.

Once all that is done, turn the TV on and set it to the component input that you just connected to the 990 (this may involve pressing an "Input" button of some sort on the TV's remote a few times to cycle through the available inputs until you see the correct input label displayed on screen). Now turn on the 990, the DVD player, the satellite receiver, and the CD player.

Go into the 990's setup menu. (To do this, you hit "TNR" on the 990's remote, then the "MENU" button just below the "9".) Select "INPUT CONFIG" and then select "DVD CONFIG". Under "DVD CONFIG," change the "AUDIO" setting to "Optical1" and the "VIDEO" setting to "COMP-DVD", then select "RETURN TO VIDEO INPUTS" and go on to "VIDEO1". Set "VIDEO1" for "Optical2" and "COMP-VID1", and while you're at it you can change the input's label to "SATELLITE" if you want. Then go to "RETURN TO VIDEO INPUTS", select "GO TO AUDIO INPUTS", and pick "CD CONFIG". Under "CD CONFIG" set the audio to "Optical3." You now have all of your inputs configured. You can leave your TV on this input now and the 990 will handle the video switching for you automatically when you select inputs.

All of what I just described is covered very well in the 990's manual, which I recommend reading. The manual also can walk you through the rest of the setup process for the 990.