I am a glasscutter by trade.
You will have to make your holes half the dia away from the edges.....make sure your tank glass is not tempered (you cannot drill or cut toughened glass)
You will need bulkheads for the pipework these will be larger than the pipe so you will need to know what size of pipe you want to use, buy the bulkhead and drill the hole to accomodate the bulkhead fitting
Solvent weld pipework and fittings
Do a little research into types of overflow.Durso and otherwise, be aware that your overflow will suck down air along with the water and can end up sounding like a flushing toilet 24/7...