What's the best way to plant raspberries?

Raspberries 'sucker' and once growing away will spread out to fill any available space, so it doesn't really matter if you plant one at 18 ins or six in a square foot (apart from the waste of new plants). You shouldn't let them fruit the first year anyway as you want to build up the strength of the canes. After that, you need to thin the canes sufficiently to allow light and air between them and for the practical purpose of picking them! Far more important is to prepare the ground properly as they will be there for several years, so make sure you have eliminated all perennial weeds and dug in some compost into the soil with some bonemeal or other slow organic fertiliser. Mulching beween the canes next summer with some well rotted compost will help suppress weeds and keep the moisture in, which will give you an excellent crop.