Depends if you are male or female.
If you are a girl then your approach to the person concerned can be a little more direct as subtlety is usually wasted on a bloke.
The opposite holds true if you are a bloke but I suspect that she may already know if you have regular contact with her. Either way on any "chance" encounter let your personality shine through but be confident and cheeky when you pluck enough courage up to let her know you exist.
For both sexes you will need to find out if the person is actually available. Befriend a known friend so it becomes a mutual one somehow.
But the most important thing to consider is how long will it remain a crush?