How can i Flush the Cooling System on Vauxhall Vectra 95 Reg ?

I drive Vauxhall Vectra 95reg and from past few days coolant had started to leak into the oil resulting in a mayo looking film forming on the oil filler cap and engine gets heated very quickly. Took to the Local garage who advised me that it is the Head Gasket fault and need replacing and its going to cost a fortune. One of my mate has advised me to try steelseal.co.uk and see if that helps. Inorder to use it i need to Flush my Coolant. Has anybody got anyidea how to do this.