Step 1: Prepare the compost and liner

Never use garden soil which is too heavy, badly drained and lacking in nutrients. Always use new compost - many plants are packed into a small area and they need the best conditions available. Pick a compost without soil, although you can add a small amount of loam-based compost improve fertility.
Add water-retentive granules to the compost. These swell up into a glutinous mass, acting as a water reservoir, and claim to reduce the need to water so often. Incorporate slow-release fertilizer to reduce the need for extra feeding later.
An open wire basket needs a liner to prevent compost falling through. Traditionally, sphagnum moss was used, though many substitutes are available and work just as well, such as ones made of coconut fibre, or you could even use an old woollen jumper!
For a cheap liner, try a black plastic bin liner inside a thin layer of fresh grass cuttings. These start off green but soon turn to a straw colour and look fine even if they are not covered by trailing plants. Add some drainage holes a few inches above the bottom of the base to create a small water reservoir.