how & what to do before planting garlic & onions in september

Dig over the bed and dig in a lot of well composted manure or garden compost. I am assuming you are planting 'sets' not seeds...

obviously as you are digging remove any weeds that you come across.

Make sure the planting bed is freely draining - no puddles of mucky soil. Lot's of composted manure as stated above. Add sand or grit if it's too compacted.

If the soil has been under pavement for a while, you also may need to add some mycorrhizae, especially Glomus species. Members of the onion family are dependent on them.

The deeper that you can loosen and prepare the soil the better. Onions and garlic roots reach a depth of 12 to 18 inches.