OK, I can tell you how not to do it and then hopefully you can work out how to do it well.
1) Out of the blue say you need to talk – without having given any warning signs that things might not be as you would like them to be.
2) Sit them down and say you want to go on a ‘break’ – promising you will come back to them in a couple of weeks but saying you still want to be friends the whole time.
3) A couple of days later kiss someone else while out with you semi-partner.
4) Officially break-up with them.
5) Same night while staying at your ex’s house after a party sleep with your new partner.
6) Flaunt your new relationship in your ex’s face insisting of being friends with you ex.
7) Keep telling you ex that you love them and just need space, while dating someone else.
8) Tell your ex every detail of your new relationship because they are your best friend.
So, that is how not to do it.
I would suggest the best way to do it is:
1) Tell them it’s over.
2) Give them some space.
3) Contact them after a month or so to initiate a friendship, but be prepared for them to maybe tell you where to go.