Parsnips like rich, slightly heavy soil, well dug but NOT recently manured (as, like carrots, they will fork if the soil is TOO rich)
As soon as you can dig the bed over and produce a fine tilth, the conditions are fine for planting parsnip seed in the ground ( if the weather allows you to get a fine tilth, its warm and dry enough!)
Parsnips take a long time to grow BUT you can get a worthwhile crop even if they are sown in late spring. MAKE SURE you use fresh this year seed, because parsnip seeds do not keep well. If you HAVE to use last years seed, pre sprout it to check for viability ( put on damp kitchen paper and watch it sprout, then snip the paper up so a bit has a sprouted seed on it and then plant the paper)
Follow the instruction on the seed packet about how/where to sow, and you may as well be generous as the seed doesn't keep well....
I plant some outside but at least half of my parsnips are planted in loo roll tubes
Lightly cover the seeds with more growing medium and water well.
Transplant into their final position in the ground AS SOON as the seed has germinated ...If you leave it too long the tap root emerges from the bottom of the tube and, when transplanted, may be damaged. This won't hurt the plant BUT you will get a smaller, forked root!
So, make sure you have the parsnip bed ready for planting, if you decide to follow the "tube" idea!