First you will need a pudding basin with the right capacity. Measure this by filling it with water from a measuring cup. If your basin is too small, your pudding will expand right out of it.
Next, you need a large saucepan with a tight-fitting lid. The pan should comfortably hold a trivet or upturned saucer with the basin on top, with space for the lid to fit properly. You can use a steamer to cook puddings, and collapsible metal vegetable steamers can have their handles unscrewed so that you can stand a basin on them. Mini puddings can be cooked in a bamboo steamer or bain-marie in the oven.