If the winding for the filaments on your transformer has a centre tap, than connect that centre tap to ground. If the winding has only two connections, than make a connection between one of them and ground.
On the site of The Valve Wizard you can read some more on the subject, like how to make an artificial centre tap (which by the way doesn't seem necessary in your amplifier because the level of signal in it will be relatively high, unlike in a guitar amplifier).
http://www.valvewizard.co.uk/heater.html