I would start now but follow this or something similar: https://www.gardenersworld.com/what-to-do-now-september/ ...or look for further tips depending on the size and aspect of your garden, and divide the work up into smaller portions so that you don't do anything too hasty. There might be some Autumn and Winter growth to be enjoyed, whilst those areas which are cleared now might be right for planting this side of the colder months depending on what kind of result you're after.