Best Answer - Chosen by Voters
Firstly, Good for you, this sounds like a great idea, and your sure to have a good time. Andorra is right in the mountains, so it's going to be very cold, that means you need to pack accordingly, lots of jumpers, sweater, thick trousers, etc...You may need to invest in a another winter coat preferable 3/4 length.
As for flying, if you've flown before than really there is nothing to it, just remember that you'll need all your travel documents, including passport, tickets, hotel booking details, etc... on the currency side, you need to ensure you have at least the 400 Euros upper limit you suggested, maybe more, which will depend on meals, if you need to hire ski gear, etc...
Have a great trip