There are a huge variety of ideas and recipes for homemade plant foods and fertilizers available, and different gardeners swear by different recipes. To make sure that a particular plant food recipe will have a good effect on your specific plant and soil type, consider things like soil pH, water uptake, drainage and your use of other fertilizers.
Variants on using coffee as a plant food are widespread. You can reuse old coffee grounds by sprinkling them lightly on the top of the soil in plant containers or around the base of plants. You can pour old, room temperature coffee around the base of plants as liquid fertilizer, or some gardeners suggest you should dilute it with several parts of water to make the coffee's effects gentler. Or, if you don't want to add water to the soil, let used coffee grounds air dry on baking sheets or flat trays and sprinkle them on soil for a dry fertilizer. Coffee is an acidic substance, so it's great for alkaline soils, but also for neutral soils, since most plants prefer soils that are very slightly acidic. You can do this once a week with acid-loving plants like azaleas, blueberries, rhododendrons and roses, or less often with other plants.
Epsom salts are a popular ingredient in homemade plant foods, because they often add needed minerals--magnesium and sulfate--to the soil. Mineral fertilizers can add a significant growth boost to many plants. Add a tablespoon of Epsom salts to a gallon of water, and water with it once a month. This is especially good for houseplants, roses, potatoes, tomatoes and peppers. You also can mix Epsom salts with ammonia for an all-purpose plant food good for houseplants and lawns. Use a cup of each, and mix with enough water to fill a quart container.
Eggshells are made of calcium carbonate, which stands in for lime with houseplants or in organic growing situations. They make a great home fertilizer with some preparation. You can let the eggshells dry and then crush them into a dry powder, then sprinkle it into soil that needs an alkaline boost. Or, for a liquid eggshell mixture, you can soak the eggshells in water in a sealed container, and pour the water around the base of plants once a month or so. However, this method can get smelly, so be warned.
Vinegar as a plant food works along the same lines as coffee does. It also is acidic and has several trace minerals that enrich the soil. Add a tablespoon of white or apple cider vinegar to a gallon of water and water plants with it once every three months. It helps counteract the effects of alkaline tap water, and works especially well on acid-loving plants.