Need basics for joining two domains into one forest

I've worked with AD for several years, but I've never had to merge two
disparate domains together. My company just purchased another, which
is how these things always start, so I need to bring this second
company's AD structure under my own. My idea is to merge the two
domains into a single forest, then once that's stable, to make the
second company a subdomain of the first.

Are there any good online resources anyone can recommend on how to do
this? I also have to merge their Exchange with ours. Unfortunately, I
don't have a lab that I could run first, so this has to be right the
first time. Thanks!

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