Before you fertilize, determine if and what nutrients are needed. The best ways to tell if your soil needs anything are to observe your plants and to test the soil.
Soil tests are especially useful in new gardens.
Once you have gardened in a place for a few years and brought soil fertility up to snuff, you can get a feel for what to add each year without testing. This rate is for plants requiring relatively high fertility, such as annuals and vegetables. Perennial borders generally need fewer nutrients, and native and wildflower plantings require even less. When I fertilize, I just sprinkle the material on the soil surface. Rainwater or irrigation water will eventually wash the released nutrients into the soil.