I found this site with the directions you would be looking for:

http://www.toronto.ca/union_station/dire...

Directions to and from Pearson International Airport by public transit

There is no direct public transportation at this time between Union Station and Pearson International Airport. However, GO Transit, the Toronto Transit Commission (TTC) and Airport Express offer regular service between various points in Toronto and the airport.

GO Transit operates bus service from York Mills and Yorkdale subway stations on an hourly basis, every half-hour during peak periods. For more information call GO Transit at 416-869-3200 or 1 888 GET ON GO (416-438-6646), or visit their Web site at www.gotransit.com.

The TTC's 192 Airport Rocket route is one of three routes operating to and from Pearson International Airport. The 192 operates every 10 to 12 minutes from Kipling Station, using a fully accessible low-floor bus, stopping at Dundas Street West and East Mall Crescent. To reach Kipling station from Union Station subway station, take a northbound train to St. George subway station. There, get on a westbound train to Kipling station. Regular TTC fares apply with full transfer privileges to and from other TTC routes. For more information on other TTC service to and from the airport, call 416-393-4636 or visit their Web site at www.ttc.ca.

The privately operated Airport Express bus operates between major downtown hotels and Terminals 1, 2 and 3 at Pearson International Airport every 20 or 30 minutes. For complete details, call 905-564-6333 or 905-564-3232 or visit their Web site at www.torontoairportexpress.com.

Grab the bus - the airport bus (see the signs outside the terminal - you can get your ticket at the booth) will take you right downtown to the Royal York Hotel (across the street from Union Station). Don't bother with pricey taxi cabs.

so take the 192 to the kiping station

then take the bloor danforth subway to spindia

then take the yonge subway to union srtation

to me fining it on the website was very easy

Just take the shuttle bus http://www.torontoairportexpress.com/

I've never used it as I hate buses or any form of public transportation but it seems to be the easiest way to get downtown