Late summer is a good time to get carrots in - the soil has plenty of warmth, probably isn't too rich because it's been working hard for you growing spring/summer veggies, and hopefully autumn rains and cooler temperatures aren't too far off.
This year I'm planning to grow St Valery - an old French variety with long orange roots - and Purple Dragon - a purple-skinned variety with long thick roots.
Before I hit on the following method of growing carrots, I used to have problems with non-germination and with uselessly forked carrots. The last couple of years I seem to have got it right. Here's what works for me:
1 Choose a bed that hasn't had a big feed of fertiliser or compost recently - rich soil can cause problems with forking.
2 Make sure the soil is well cultivated without rocks or any other big chunks of hard stuff. Remove any mulch from the surface - you want a fine tilth to sow your seeds into.
(Forget about planting carrot seedlings, you really need to grow carrots from seed because they have a long fragile taproot which after all, is the whole point of carrots!)
3 Unless the soil is already moist, water the bed then leave for an hour or so.
4 Make a wide, shallow drill (no more than about 1 cm deep).
5 Sow your carrot seeds into it in a zigzag pattern. They're fiddly little things, but don't stress too much about spacing them exactly. Just try to avoid any big gaps or big clumps.
6 Cover with a fine dusting of soil - your seeds need to be covered, but not too deep. Small seeds need to be sown near the surface.
7 Water in - gently.
8 Cover with a layer or two of damp newspaper. This is to stop your seeds drying out before they germinate, and prevent the birds from scoffing the lot. Weigh down the sheets of newsprint with something long and fairly heavy, or they'll blow away. I use garden stakes pinned down with weed mat pins.
9 Water once a day until your seedlings are up.
10 After a week or so, lift the edge of the newspaper each morning to check whether the seedlings are up. If so, immediately remove the newspaper.
11 Continue to water regularly until your carrots have enough top growth to shade the soil. You could also mulch at this time.
12 Don't bother too much about thinning your carrots: just start to pull and use them when they're still small, and the survivors will expand to fill the available space.