Try to have sorted out the admin - where each XP will be living, what the initial contact pattern will be - before telling the DC, because it will be far easier for them if you have answers when they ask about those sorts of things (and unless extremely young, then they will)
Tell the DC together, with lots of reassurance that you both still love them very, very much. Do this before news if the split becomes common currency (ie confide in only one or two totally trustworthy people, or a paid counsellor) until that point.
Brief the school/nursery/whatever first working day after you tell DC, and update ememrgency contact lists. Sort out how to handle school communications between you.
Use an app like Time Tree in which you can have shared calendars with XP (one for him, one for each DC, one for you, all shared: you can add private ones for you as well) as everyone can see the updates straight away.
Get any key messages in writing - eg an amicable email 'Hi, just confirming that we agreed you would do an extra pick up from school in Thursday and take them straight to the dentist. I'll be in by the time you've all finished and you'll give me the £30 for Cubs subs when you drop off. I'm doing an extra pick up next week on Tuesday - enjoy your training session that night!'
Live as close to each other as possible, and let th DC free range once they are old enough to cope.
Decide what occasions you can handle together - will you ever go to his family events or vice versa? But be ready to facilitate DC contact with both sides if the family if ever necessary.
Always pick up if it's your DC calling, but perhaps set times for when other parent can call DC at their other home (eg call every day if you want to, weekends first thing up to 10:30, any day evening between 6-7 - aim is maximum freedom, but within a predictable pattern IYSWIM)
Always listen to requests for flexibility, and say yes unless it's impossible.
Have a planning meeting about once a term to sit down and discuss any changes to the DC's needs and preferences, and forward planning for the next half term and holidays, and any forthcoming special events. Also revise budgeting.