My eldest child (nearly 8) has always been homeschooled. When my daughter arrived in December I worried and wondered and couldn't figure it out at all at first. For me it's actually far harder to keep the middle child (DS#2, nearly 4) occupied. DS#1 has many things he enjoys doing and the baby just does whatever we do and is usually quite happy. Preschoolers are a bigger issue for me.

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When I was planning ahead, people told me to do the homeschooling when the baby napped. Well, she doesn't nap all that much, and it's almost always in arms or in a carrier.
Usually DS & I will talk about what we want to do with our day, whether he wants to do computer work, or build, or draw, or paint, or read books together, or just play with his brother.
Somehow we get it all in there, especially with nature walks and library/museum visits. The DS#2 is learning by osmosis from whatever we do together, so I don't really think or plan much for him educationally speaking...he's definately purely unschooled (as I personally believe all children should be until 3 or 4 or until whenever they show you they are ready for something more...)
So I guess I'd say that I just try to provide them a rich learning environment and go with the flow. We aren't really the curriculum-based types, here, so it works for us and the baby isn't really that complicating to the whole thing.
One last thought, since you have months to go, I'd recommend you start working towards independent activity development with your older child. Since my 2 boys had each other to be with/play with I didn't have to do much in the first month or so after the baby had come. I just sat back and watched them being brothers together. It was nice...but not what everyone has to look forward to.
I'm rambling now...
blessings,