You find the right area of each agency's website to initiate a freeze. IIRC you have to make an account with them. You will have to prove your identity, which involves answering multiple choice questions about loans and residences you've had.

My hang ups were mostly that I have a terrible memory so I had to look up info on loans to answer those questions. I was also full of rage that I have to give them my current info, but they 100% already have that from my bank, so whatever.

When thawing you log in to the account and choose what days you want it thawed for. If you apply for loans or lines of credit it's smoother to do ahead of time, but IME nobody cares if you have to thaw and then reapply.