choose a liner that will hold moisture, the coir and moss ones look good but water evapourates from them quickly
buy a mixture of trailing and upright bedding plants (garden centre will advise) lobelia and surfinia are pefect, and some water retaining gel crystals, and any compost will do
start with a two inch layer of compost in the bottom of the basket. poke holes in the sides of the liner and stick the root ball of the plant through so that they lie on the surface of the compost.
cover with more compost. three plants per layer and three layers should do it. in between each layer sprinkle some gel crytals. put the trailing ones in the sides and the more upright ones in the top.
or if you cna't be bothered with the hole poking thing just plant them all in the top
you don't need many plants. water every day, even if its rained, as the rain won't penetrate the plants. a hose with a long hanging basket watering-thingy is great for this job. give them loads
always water in the evening, or when the plants are shaded to avoid shorching the plants
you can have fun colour coding and green foliage ones also look great
hope this help! enjoy!