Agree who is dropping the kids up or picking them up, and what time.
Agree what will happen if your ex wants to have the kids at additional times, or you want him to have them on an odd extra (eg if you want a night out, or work late)
Agree what will happen if he doesn't want the kids on one of "his" weekends - ie do you want to agree that you'll try to swap for a different one?
Basic rule we've found works well is that we can each ask to change agreements whenever we like, and the other person can say yes or no without pressure and choose whether they want it to be a swap or just a one off change to the routine. We also expect the other to help out if at all possible if the change is to support work comitments.
Work out what belongings your ex should provide for the DCs when they're with him, and what things will travel with the DCs. It's easier for everyone if most things are provided in both houses, and that's easy enough to do when they're little.
We agreed a monthly meet up to discuss which nights ex was having kids over coming weeks, and also discuss anything else to do with parenting. It's less frequent now (maybe every 2 months) but it's still good to have a little bit of time set aside to be parents together. We discuss things relating to schools, behaviour, friends, etc.
In terms of is your 1 year old ready - my DD started doing overnights at her dad's when she was about 9 months and that was fine, but we did 1 night a week for the first year or two and only switched to alternate weekends when she was a bit older. You might want to try one night at a time at least for the first few weeks to see how she goes. She'll probably find having the older siblings with her a big support, so may cope fine once she's used to his new place. If she's attached to her dad, she may find nearly 2 weeks away a long time too - could he have them for the evening or for a night in the week?