hi andrea, welcome to the forum
as well as what has already been said, you may notice her being more affectionate a day or so before they arrive. she may also be restless...lots of pacing, not being able to make her mind up what she wants to do.
it is difficult to give you a clear idea as they are all different & have their own way of doing things
when the babies are ready to arrive, she will lose a little "plug" from her rear end...a fleshy coloured bit of gunk lol, except you might miss this if she cleans up after herself quickly...there may be a little discharge & her back end will have opened up compared to normal.
also, the contractions will be obvious...big shudders down the length of her body & she'll probably lay down & start to push.
it's all very exciting! do feel free to ask away if you're worried about anything, & i'm looking forward to hearing all about the bubs when they arrive!