start in pots end of this month and plant out when they have two proper leaves.. in good well dug soil sunny spot with some blood fish and bonemeal dug into the soil or pelleted poultry poo , give them some protection til they establish and remember to give enough water
.....and protect from snails, sunflowers are their most favourite snack! I put the transplanted seedlings in very large pots, one in each, and put gravel all over the soil surface to stop snails.
I'd also suggest staggering the planting to give a longer season of sunflowers. I poked a few into the ground this weekend just in case they survive, and will plant more at randon through until end of April in the hope of a varirty of flowers through the summer.