I did exactly this a couple of years back, 9x13 for an 8x12 shed.
Digging out the area was the hardest - and that's a fair bit of soil to dispose of make sure you know where it's going!
You can dig it out over a period of time and put in the hardcore but you really want to lay the concrete all in one day.
You need another person to help you tamp down the cement and probably to hold the barrow as you pour.
It's not actually that difficult but you want to make sure you put the shuttering in level and tamp the concrete down level to that because the shed won't sit level otherwise.
*It is however hard work*
You will be lifting several tonnes of sand and cement up to put in the mixer, pouring the same weight plus water into a barrow and then taking the barrow to your pad and pouring it - and I'd expect that you'd spend most of the day doing that.
But it does give you a really good base way better than you'll get with slabs.
Other advise - even if your shed is ready coated give it another coat of waterproofing on the underside of the floor - you might even want to glue some dampcourse strips to where it touches the pad - you'll never get to these places again.
you also might find this site useful
http://www.readersheds.co.uk