Ok iam not a expert but by on the flat means not in a trench , So the
steam keeps clean , Use a pipe or something similer to create the
blanch. Here is a post from a chap on a forum that i use now and again.
Might be helpfull.
Planting outdoors:
Pot leeks should have 7 to 10 days hardening off in a cold frame before planting out. About a week before planting, rake 50g plant food powder per sq metre (per 1 sq yd) into the surface of the soil. Pot leeks are planted with soil up to their lowest flag whilst Blanch leeks are planted so that their bases are about 10cm (4in) below the surface. Make a suitable hole for each plant and sprinkle in a little plant foodpowder to encourage the roots to penetrate the soil. From now on the soil is never allowed to dry out. Root and foliar feed once a week with full strength plant foodfrom the end of June onwards. Regular feeds are essential for actively growing leeks; the high potash content of plant foodkeeps the plants robust.
Lengthening blanch leeks:
When the plants show about 20cm (8in) above the surface, place a cardboard tube about 10cm (about 4in) long and 25mm (1in) in diameter over each one. This is replaced with a standard land drain tile 30cm x 9cm (12in x 31/2in) when the plants reach 45cm (about 18 in) in length. By improvising with tubes of various sizes, the length of the leek can be slowly increased. Never cover the uppermost third of the plant; this only weakens the growth.