To register as a write-in candidate in Illinois, first download
this form.
Fill it out (don't sign it yet!) and have it notarized (sign it in the presence of the notary).
You'll need to include some information specific to your county; you can look that up
here.
This page will also tell you the address to mail the completed form to for your county.
I put a sticky note on my declaration, asking for an email confirming that the form has been received and properly filed.
They don't have to pay any attention to sticky notes, but it doesn't hurt to ask.