Okay given the size of the game you'll either need multiple CDs at hand or one DVD. I hope these instructions work, it's quite some time since I used Steam's backup feature - so here goes (it's not as complicated as the number of steps might suggest)
1) Put your disc (preferably a DVD but if CDs are all you have at hand that will work, though you'll need several) in to your computer's disc drive. Cancel any windows that Windows pops up on screen asking if you want to burn music or some such.
2) Load Steam
3) In the Steam program look to the top of the screen, it should have the menu options Steam View Friends Games Help.
4) Click Steam, the menu option at the top of the program window.
5) Click Backup and Restore Games
6) Click "Backup currently installed games" and then click Next
7) Find Ship Simulator Extremes in the list, click the checkbox next to its name (so a tick is present) and then click Next
8 ) Next you should be seeing a window that asks you to choose the place where your backup will be made, click Browse, near the top of this window is a drop down box - clicking it brings up a list of hard and disc drives attached to your computer. Click the drop down box, find your disc drive in the list (the one containing the blank disc) and click the drive's name. For instance, if your disc drive is assigned the "D:\" address in My Computer then D:\ is what you'll need to choose.
9) This should bring up a list of folders on the new disc you've inserted - there should be no folders listed however, as it's a blank disc.
10) Click Make New Folder (if the option is greyed out, click 'D:\' (or whatever your disc drive is labelled as) first) and type SSE (or any name you want). When you've created the new folder, make sure it's clicked, then click Select.
11) Click Next; it'll ask you for the Backup File Name - this can be any name you want but calling it something like SSE will make it easier to identify should you ever need to reinstall the backup.
12 Under where you've just entered the file name is a drop down menu "File size". Here you can choose how Steam will backup the files; ie. whether you want to back up the game to a CD, DVD or hard drive. Click whichever option is relevant to you. If you choose "CD" it will tell you "Approximate number of compressed files which will be written" - this essentially means "How many discs you'll need"
13) With the file name entered and the file size selected, click next.
14) A message along the lines of "Please wait while the backup archives are being compressed and saved" will appear. If you are backing up to multiple CDs you will be prompted to insert new CDs when the time is right.
15) When the process is complete a message will appear "Backup is now complete". At this point the backup is - as the message suggests - complete and it's safe to remove the disc/s from the computer. The game is now backed up.
Should you ever need to restore the game from the backup disc you've just made you would insert the disc in to the computer and using the Steam menu option you'd pick Backup or Restore Games but this time you would click "Restore a previous game" - and follow the instructions.
I hope this helps, I'm sorry if they're not as clear as they could be, it's late and I am a little groggy
