My top three favorite DIY fertilizers are eggshells, coffee grounds, and banana peels.
You can mix a big scoop of the spent coffee grounds into your soil when planting a new plant or mixing water and pouring over your houseplants.
The laziest way to use a banana peel as DIY fertilizer is to place it on top of your potting soil and cover it with a bit of the dirt.
Of course, to get the most nutrients out of your DIY fertilizers, you can mix the eggshells, coffee grounds, and banana peels together.