Day one has a strange energy. Part of you feels determined, maybe even a bit proud, and another part keeps checking your phone like something is about to happen. Both can be true at the same hour.
For a lot of people the first day is less about sadness and more about restlessness. You keep reaching for the phone out of habit. You draft messages in your head while doing other things. You might feel a pull to send one "clean" final message to explain yourself. That pull is normal, and you don't have to act on it.
What usually helps on day 1
Keep the goal tiny. You don't have to commit to forever today. You only have to get to bedtime without contacting them. Tomorrow you can decide again.
Make their chat harder to reach. Archive it or mute it so it isn't sitting at the top of your messages. If you check their stories, mute them too. You can undo all of this later, and nobody gets a notification.
Write your reasons down while you still remember them clearly. Three or four lines is enough, like how you felt waiting for replies or the conversation that made you decide. Tonight, when your memory starts softening things, you'll want those lines in front of you.
Tell one person. A friend who knows you're doing no contact can be the one you message at 11pm instead.
If you feel nothing at all
Some people feel oddly calm on day one, and then it hits on day two or three. That doesn't mean something is wrong with you. Shock and relief can both feel like numbness. Eat something, drink water and try to sleep at a normal time.
If you already slipped today
Then today is day zero, and tomorrow is day one. Don't send a follow-up to explain the first message. Put the phone in another room for an hour and start the count again in the morning.
Tonight, before bed, write a line or two about how today went. It doesn't need to be deep. Something like "wanted to text at lunch and at 9, didn't" is plenty. In a few weeks you'll be glad to have a record of where you started.
Getting through the first day is the part most people find hardest to begin, and you're already doing it.