[eluser]Barouma[/eluser]
It is not possible to customize your 404 error page if your web host has not enabled this facility for your website.
If your web host has this facility, you will usually find mentioning of this information somewhere in their documentation. In fact, if they mention somewhere that you can customize a file named ".htaccess", it probably means that you can also customize your 404 File Not Found error page.
The steps in customizing a 404 error page are given below.
Step 1: Create a customized 404 File Not Found Error page.
Step 2: Create a .htaccess file
Step 3: Upload both files.
Step 4: Test the page.