Worm castings are one of the best all-natural fertilizers you can supplement your garden and indoor plants with. It can be used like ordinary (aerobic) compost and you can’t over do it. Worm castings will not burn up your plants. You can dig it in lightly around your plants in the topsoil or you can use it as a soil amendment. Worm castings are packed full of nutrients which allows it to go twice as far as the aerobic composts, so use half as much. If you are starting a new bed, I would suggest spreading the castings at a depth of 3 inches and till it in.
For plants in pots and containers, I suggest mixing 25% worm castings into your potting mix. This is about ideal, but experiment. It may vary according to what you are mixing with. Worm castings are great for starting seeds. Just a little tid bit: Worm castings contain 5 times more nitrogen, 7 times more phosphorus, and 11 times more potassium than ordinary soil.
Worm castings can also be used as a compost tea. Mix two tablespoons of castings with a liter of water and let it stand for a day, shaking occasionally, then water under the plants. (One liter water bottles make a good watering container if you drill a few small-diameter holes in the lid.) Compost tea is great to use when transplanting. Using a slightly diluted version of your compost tea will help prevent transplant shock if you water in your plants with it afterwards.