Clear the day, warn friends/family of the event so you can have your space.
Plan how you will handle the body, if you plan to bury him have the hole dug.
Discuss this with your vet, understand the procedure and what to expect. Ask how he would prefer to handle things.
When everything is done give his mother time to mourn and say good bye. She needs to spend time with his body.
Know that this is not your fault, no part of it is. You CAN NOT blame yourself. Know that letting him go is the most selfless thing you can do, you are putting his needs and welfare above your own emotions.
Do not let ANYONE make you feel bad for the choice as it is always better to end things a week too early then a moment too late.
And then cry and scream and weep and generally be as big as an emotional wreck as you have to be. Because if you don't let it all out you will bottle it all up inside you and you can't go on. Take some time to look after yourself but enjoy the beautiful babies that are still on their way for you.