Go for leaves that are bright green and fresh looking, with no brown discolouration. Stems should be firm. Unlike many vegetables, larger Swiss chard leaves aren't necessarily tougher than smaller ones.
The leaves and stalks should be cooked separately, or the stems cooked a few minutes longer than the leaves, as they are firmer. Wash then cut the stalks from the leaves and leave whole or chop, as required. On some older leaves, you may need to cut out the ribs too. Leaves can be left whole or chopped, as required.
In the fridge for up to four days.