Flea beetles chew tiny holes in soft-leaved plants, in my case, tomatoes. You might catch them jumping as you move through your garden. Yellow aphids attacked my pepper plants. The leaves were yellowish and very sick looking, curling at the edges. Looking closely I could see the aphids. Slugs, you may catch skulking through the garden, or you may see wide holes chewed in leaves.
Aphid solution: trim some tomato leaves, about 2 cups (packed). I trimmed mine from the suckers or bottom levels of leaves. This encourages the tomato plants to grow up. Blend the leaves with 2 cups of water and leave over night. The next day, strain the pulp, and waterdown the green solution in a 1:1 ratio. Put into a spray bottle and spray the plants suffering from aphids. Tomato leaves contain alkaloids which are deadly to aphids.
Flea beetle solution: make a smoothie of everything flea beetles hate, including but not limited to: raw onions, garlic, chilli peppers (I used sriracha sauce), and fresh mint leaves. Leave overnight, strain the pulp, put into a spray bottle, water down slightly, spray affected plants.
Spray when the plants are dry but not before watering. For aphids, do not rinse off the treatment, water at the base of the plant to avoid this. Reapply daily until you run out of spray. Keep an eye out for future aphids. For flea beetles, I found spraying the tomatoes and chard on a wide mist, allowing the spray to get all over the place (including a strange perfume on my skin), deterred them immediately. Note that flea beetles are not life-threatening to established plants.
Slug solution: save the shells from eggs. I leave mine in an open plastic container on the counter. This way the eggy part can dry. Crush them and sprinkle around the base of plants. Slugs don’t like slugging over jagged egg shells, for obvious reasons, and the shells also feed the soil.