So lets say you found a good composition and you got all the gear you need and perhaps even more than that because you took my bad advice :-)
Set the camera on Aperture priority, a high f-number like f/16 to f/22 and ISO 100. You want the picture to be sharp all the way through.
I like to set the white balance to "Cloudy" to get a bit more warmth in my shot.
If you use a polariser, this is a good time to screw it onto your lens and adjust it. If you want to keep the reflections in the water, leave the polarizer in your bag. Maybe you should leave it there anyway if this is your first try.
Hold the camera towards the sky to measure the light level, and then on the foreground. The number of stops tells you if you should use a light, medium or strong ND grad filter. If you have no clue, use the medium filter.
A video tutorial can be found here.
Set up the camera on the -pod of your choice. The reason I prefer the gorillapod is because it will hold the camera lower than my tripod will.
Adjust the polariser if you are using it. If you are using a wide angle lens and you have a clear sky, leave the polarizer in the bag or you will get a dark blue V in the sky in the middle of your picture. Well - at least look out for it.
Mount the adapter ring and the filter holder. Slide the filter in and adjust it to fit your horizon.
(Well, your camera is so low now so you should probably fit the angle finder first unless you want to lie on your knees on the ground.)
Do not forget the Hotshoe spirit level unless you like straightening your shots in Photoshop afterwards.
If you havent set your timer yet, do it. The wireless remote works with the timer.
Some magazines suggest you use the mirror lock-up functionality to reduce movement even more. On the 400D it is found under the Custom Settings menu, option 7. It will lift the mirror when you press the shutter, then wait for 2 secs before it shoots the picture.
But I have also read that the mirror lock-up is only necessary when you shoot macro's so you can leave this option out for now.
The focus should be set approximately a third into the picture, then switch to Manual focusing so it won't be disturbed. If you have a foreground interest, set the focus on that. If it is easier, use your lowest focus point. Auto-focus is out of the question.
Well this would be a good time to press the shutter. Unless the sunset is gone :-)
I now recommend checking the histogram and highlights clipping and adjust the exposure down if you have any burnt out areas.
Well this is how I would do it.
You can find a video tutorial for traditional wide andle landscape shots here.