When planting Crocosmia, always plant the corms at least 4" ( preferably 6" for the bigger corms, such as Paniculata ) below soil level. The reason for this being, the hardest frosts only penetrate 3" or 4" below the ground, so your corms will not be affected.

A dressing of a good general fertilizer such as Vitax Q4 or Fish Blood & Bone at roughly a handful per square yard is recommended in early Spring.
Avoid high Nitrogen fertilizers such as Growmore because they will just encourage leaf growth at the expense of flower growth later in the year.

Normally, once plants are established additional watering other than that supplied by rainfall is not required.

When the plants have finished flowering, cut back the stems, but leave the leaves on even after they have died back. This will make good insulation against frost in the winter months.

Division of plant clumps is best done in late Autumn and then replant the strongest corms immediately, because during the winter period, although there is no sign of growth above the ground, the corms are making new shoots below ground, and if you divide the plant in Spring the delicate shoots can easily be damaged.