If you are looking to create a transaction with, say 100% GST, we have an article here outlining the steps.
If you are needing to change to a different GST amount, e.g. 15%, you may need to manually calculate this using similar steps to the article outlined above as QuickBooks is not currently set up to allow custom tax rates.
Maybe I was not clear.
I understand how to use the default GST calculations, but in some cases, eg Telstra, often the GST is NOT 10% - how do I record a specific $ amount for the GST to match the invoice
Thank you for clarifying. If you are needing to record another amount for GST that is not 10% you can set the transaction to Exclusive of GST and manually change the amount to what you need.