I've never taken an inter-Europe (intra-Europe?) flight but will need to next year.

Do I book it as a stop on my USA>Europe open jaw, or make entirely separate arrangements?
Also, is Skyscanner still the preferred site for booking?

Any advice is much appreciated ~ thank you.

Book separately. I do all the time. One way intra tickets. Like a bus or a train actually.
I always always book direct.
I search on kayak mostly but never ever should you book via a third party.
Read the horror stories before don't post after.

Intra, yes. The big budget airlines such as Ryanair and easyJet (and they are huge) cannot be booked with intercontinental carriers. But those airlines have been forced to compete inside Europe and may match prices. Research is unavoidable. www.itmsoftware.com is the biggest site for intercontinental but doesn't list everything; no site does. Be sure to use a multi-destination search. www.skyscanner.com is good on the budget lines for one-way fares and does show intercontinental routes too.

http://matrix.itasoftware.com/

(not www.itmsoftware.com mentioned above)

Oops, thanks MmePerdu.

For me it depends on whether my over-the-water carrier (or alliance/related carrier) serves the intra-European segment. If so, it can often be a cheaper add-on than taking a LCC.

Check it both ways. Sometimes you can get substantial savings by booking a multi-leg flight. We one did a four leg flight with two legs in Europe and it was much cheap and more convenient than booking local one way flights for the European portion. Price it out. The only way to know.

One last question = when to book?
Is it cheapest to book earlier?

Our plans are pretty well set so I know which flight I want.
Thanks again~

Yes, book as soon as the flights are available for cheapest fares. The later you wait the more you'll pay as the cheap seats sell out.

Thanks very much, Mme - I just booked and it said "last two tickets at this price" which I thought probably was an advertising ploy, but when I checked again, tickets were $11 more each.

Someone should buy you a drink!

I think so too! Thanks for the thought, nyse!