If you’re drilling holes to thread string through then place another piece of wood under it and use a small drill bit first to reduce the risk of the wood snapping. If you don’t have a drill then simply wrap the string/twine around the wood three times and using the end go under the last loop to keep it tight.
Before you start gluing the collage I would try different arrangements and only glue once your happy. Also try different backgrounds, maybe glue sand to a board and then wood on top of that. If you think that the glue might show in places then sprinkle some sawdust on top or mix it in, this allows the glue to blend into the wood.
With mirrors you need to glue onto the mirror frame and not the mirror its self as it wont stick. Try and overhang the wood onto the mirror so that the frame cant be seen. And if your going to use shells and starfish like in the photo above then please only pick up sundried pieces and don’t take more than you need.
I would suggest using lights instead of candles for your driftwood, candles need to be monitored and ideally you want to have a metal layer between the candle and wood. You can buy cheap rope style lights which can simply be wrapped around the wood and hung. If you do use candles then make holes using a hole saw and slowly chip out the inside with a chisel or flat head screw driver. Only use candles with metal outings or even better is glass holders.
Sometimes just the wood on its own is so unique that it just needs a clean and a stand. Another thing to do is cut away the bottom of a piece and see the grain. You might strike gold with a lovely hardwood, if that’s the case then try and saw the wood in half and give it a good sand and oil.