Go to Register if you haven't already created an account.
Download CGMiner from here.
Create an empty text file. If you don't see the file extension (.txt) at the end of the new file like "New Text Document.txt" you have to change your folder options.
Press the "ALT" key to open the following options.
Click on "Folder options..." and uncheck "Hide extensions for known file types". Click OK.
Rename your newly created file to "coinat.bat"
Right click on "coinat.bat" than "Edit"
Enter the following lines
color 02 setx GPU_MAX_ALLOC_PERCENT 100 C:\path\to\cgminer.exe --scrypt -o stratum+tcp://ltc.coinat.com:3333 -u miner.1 -p xATTENTION: Do not forget to change the file path (C:\path\to\)!
This step is very important and can require quite some time to squeeze out the last hash of your graphics card.
Go to Mining hardware comparison to see what other miners use for their graphics cards.
Some times it requires to tweak the settings even more, because every system is different.
Open your batch file again and paste the settings
NOTE: Make sure "--scrypt" is included in the parameters list!
Go to coinat.com and login.
Than go to "My Account" > "My Workers" and make sure the worker credentials are the same as in your batch file.
Right click on your batch file than click "Open" or double click it.
You should see something similar to this