You can use liquid and slow-release fertilizers to fertilize indoor plants. To use liquid fertilizer, simply dilute the instructed amount with water in your watering can and water your plant. The frequency of how often you do this, will be noted on the label of your liquid fertilizer and also depend on what type of plant you have. For slow-release fertilizer, simply mix the instructed amount from the product's label into the top layers of your soil. These applications will usually last a few months, but they do cost more than liquid fertilizer.