Now I have to say upfront, that I don't either, because when I've to grown it, I haven't experienced problems - but, in the 15 years or so I taught gardening classes (adult education at night), each year there was always a discussion on growing parsley and how hard it was.
Some people talked about germinating seed on moist tissue paper, and then sowing, others talked about watering where the seed were sown with boiling water, and there were many other tips and techniques used too.
Typically sow in April for a Summer/ Autumn crop, or sow in August for Winter use. Thin the plants to about 9 inches apart (about 22cm). As always, do follow the instructions on the seed packet.
Here are some good tips from the master himself, who has been growing Parsley for about 40 years or so:
Use freshly purchased seed - they don't keep from year to year
Raise your seed bed slightly - this will help the soil be warmed by sunshine
Sow in rows - it'll help distinguish the young parsley plants from weeds
Be patient - the seeds take a little while to germinate
Be patient - they (the seeds, not my parents ... Lol) don't all germinate at the same time, so don't panic if you only get one or two in the beginning
And most importantly, be patient.
After our round table discussions, we came to the conclusion:
If seeds are too much hassle, buy a couple of plants (preferably in a garden centre rather than supermarket) put them into two litre (or slightly bigger) pots with some good compost, tuck into a sheltered corner of the garden and wait.
Don't bring the plant home and start chewing on the leaves straight away. If the weather is anyway reasonable (i.e. weather not too cold) they will produce an abundance of foliage ready to pick for cooking.
My final tip - I regularly use parsley in Autumn & Winter tubs and containers. It's really great to use for strong fresh greenery over these months and ready to harvest in the Spring. Tasty.