It is best to prepare your soil over the winter months; you are trying to get a light soil with very little stone in it, in a sunny position. 
We are lucky at Longbarrow to have this type of soil, clay can be a problem.
Don’t add any manure as this will make the soil to rich . It is best to use a bed that has been prepared with manure in the previous season.
Approximately 2 weeks before planting you can add a general purpose fertiliser to the soil. Granular fertilisers are a good choice and available to members in our allotment shop.
Covering the soil will also help to bring up the soil temperature as well as keeping weeds down.
Planting can take place in late February or early March, it really depends on the conditions at the time, if however you go for early sow, it would be advisable to protect the crop with fleece or cloche cover.
Sow in shallow drills approximately 1″ deep on a warm sunny day and cover.
Once the seeds have developed their first leaves, thin the seedlings out, at least a couple of inches between each carrot. It’s no good being greedy, thinning out will lead to a better crop, overcrowding will produce poor results.
Good luck