Sylink , i feel your pain! I do it with trial and error ! Error mostly
http://seamless.usgs.gov/ Go there (thanks Matt) .
In the lower left there is "Seamless Viewer " click on it. The page that comes up is like an application.
There are tools on the left. You can zoom where you want .
The download tools are self explanatory i think.
I use the rectangular one but the others are what you are after i think.
There are sometimes error messages that the area i want to download is to big . Select a smaller one etc.
This page is very slow and sluggish on my side. Sometimes it seems that it doesn't do anything , but it does!
So be patient!
There are selection parts on the right side too on this page . Choose what you want there too.
After i select an area with the rectangle selection tool from the left download section a windows pops up.
If the data isn't there what you want (geotiff or so) click on " Modify Data Request " in the upper left in this new small window.
There are many more options. For me this was the important section i didn't know.
After this it isn't so difficult anymore , especially if you know what data type you are after.
You will see " Data Format :" , "Archive Format:" and " Metadata Format: " columns .
If you click on " Data Format :" in the lower part what you want to download there are Geotiff and other formats to choose from.
Chose all what you want in this window and then click the "Save Changes & Return to Summery"
You will get the previous small window. If you selected for example GeoTIFF you will see that the output is "Output Format: GeoTIFF" now .
If you click now on "Download" you will get a new small window and after a while your download will begin.
I use this above website very few , but this knowledge mostly suffices me. I hope this will help you too
