So, the most important thing to bear in mind about the "Christmas Truce" was that it was not a single event. Truces broke out independently at many different locations - they shared many common characteristics and in some cases the participants may have been aware of truces elsewhere, but in general, you're looking at a lot of independent activity more-or-less invented on the spot. There is a good scholarly survey of different aspects of this in Meeting in No Man's Land: Christmas 1914 and Fraternization in the Great War (ed. Malcolm Brown, 2007). I haven't yet read The Christmas Truce: Myth, Memory, and the First World War (Terri Blom Crocker, 2015) but it looks solid as well.
The second most important thing is that it wasn't just on Christmas - although that was the highest-profile bit and one of the few explicitly talked-about ones. Localised truces occurred right throughout the war, though as time went on, they were more commonly tacit rather than explicit - think "oh, we never shoot when they're washing" - Trench Warfare 1914–1918: the live-and-let-live system (Tony Ashworth, 1980) is an excellent survey of this. I cannot recommend the Ashworth book strongly enough - really eye-opening and worth reading for anyone with an interest in the lived experience of WWI.
Hopefully you've picked these up from background reading, but if not, never hurts to set the groundwork :-)
As a result, there wasn't a very clear geographic focus, and single events that characterised some truces (eg football, songs, burial, souvenir exchanges) were not present at others. This probably helped the growing belief in the late 20th century that it was a myth, as various accounts of it contradicted others, or came from different places, etc.
For a series of eyewitness accounts to pick from, the first chapter of Brown 2007 or one of the recent popular histories like Silent Night: The Story of the World War I Christmas truce (Stanley Weintraub, 2001) are pretty good. (Weintraub has some interesting comments of the truce as experienced by Indian units, I think, though I can't find a copy to hand).
One of the most widely-used accounts is from RJ Ames, an officer in the (British) 1st Staffordshires, and something like this would work well -
I was in my dugout reading a paper, and the mail was being dished out. It was reported that the Germans had lighted their trenches up all along our front. We started calling to one another Christmas wishes. I went out and they shouted 'no shooting' and then somehow the scene became a peaceful one. All our men got out of the trenches and sat on the parapet, and the Germans did the same, and they talked to one another in English and broken English. (...)
For an image, the canonical photo is this one (IWM Q.11745), and you could do a lot worse than reuse it.
