The CSS Profile is an online application used by colleges and scholarship programs to award non-federal institutional aid. Most often, you will see that the CSS Profile is required by private colleges and universities, though you should check the requirements of each of the institutions you are applying.
You can access the CSS profile here, and begin filling it out as soon as you have begun your college applications and have finalized your college list. This page also includes step-by-step instructions.
Every institution will have their own deadlines for when they would like the CSS profile to be submitted, so please be sure to check their websites to verify when you need to complete it.