After hunting high a low for it, as the manual is absolute rubbish, can someone tell me at what stage of published checklist am I supposed to turn it on?

In the real world C90B checklist, you'll find that the master battery switch gets turned on and off several times during the BEFORE ENGINE STARTING checklist. I can't say what's in the Carenado checklist, as I've rarely looked at it...but basically, you need the battery on to close the FW fuel cutoff valves, then you turn the battery off to test the boost pumps. Once you've checked the boost pump operation, switch the battery back on and leave it on.

Excellent Yoda

Thanks for reply.