This station is a bit confusing and currently under a lot of construction/renovation. Certain areas look like a run-down version of NY's Grand Central and Port Authority while the rest resemble a modern airport. It's Toronto's major transportation hub and Canada's busiest. I've only been here on a Monday morning, which was probably the station's downtime since there was only a small crowd throughout the terminals.
The bus terminal is separated from the trains by Bay Street. There are hardly any staff members, let alone an information desk, around so it's important to keep an eye out for directions/signs.
We took the newly inaugurated Union Pearson (UP) Express train from Union Station to Toronto International Pearson Airport. It's quite pricey, $27.50 per person but the train runs every 15 minutes and guarantees a commute time of 25 minutes. We followed all signs that pointed to the Skywalk. It's a good 5-10 minute walk since it's closer to the CN tower than Union Station Bus Terminal.