How to use a post hole digger for 300mm hole?

budget your time, take a good break as soon as you get tired, even if thats twice per hole
A 300 into clay, run your 300 down about 300 then change to an 80, those 300's are quite fat in the middle, and you have to force that down - theres no cutting tip for the shaft end - running it down 300 first allows it to stay on track, don't even think about center boring from the top, you will not enjoy the 300 trying to walk off all over the place, you'll find it easier, boring 300 with a one man is never easy but is easier with a center bore
I've recently done 85 plus for stockyards, all 300, all in clay, with a one man 58cc chinese knockoff, enjoy, small bites, lots of lifting to clear, although they do rev enough to self clear until the flutes are below grass level

consider also, measure your poles, if you can use a 200 then use that, a bag of redimix down each hole eliminated having to ram them, especially in a retaining wall scenario, but wouldn't be the first strainer or gate post to have a concrete sleeve

also consider getting a $14 shovel from mega, cut the blade off it and weld it back on so the blade is at right angles to the handle, makes an excellent 'duckbill' for cleaning the bottom of deep narrow holes, I use one all the time to clean the bottoms of 600mm holes, usually 1.5 deep, for pole sheds, drop a concrete block in the bottom on hard soil and stand the pole on that, then drop in the concrete, creates a foot and a sleeve at the same time, never fails

lol, buy a borer on here for $185 and tuck it in the shed for those other jobs later - they often come with a selection of augers from 50 through to 300

There is no way you will get anywhere with a one man borer that size.150 is about your limit,and you can only bore one hole regardless what size it is.You can't bore a small hole and progress to a larger one, it doesn't work like that.The Auger needs to follow the solid centre and be a tight fit around the edges.300 is the max even for a two man borer if you can get one,normally 250 by hand and 300 by machine.