garlic- hot pepper spray is a common homemade insect repellant/killer. It isn't so much steeped as just blended in water. Search the site here and there are lots of recipes for it. I suppose it might work infusing the garlic and hot peppers in hot water.
Real insecticides can be made from garden plants, but they carry the same warnings as commercial insecticides. I grow nicotiana, which is ornamental tobacco. Steeping those leaves would give you a nicotine solution. Being ornamental it would be much less concentrated than steeping tobacco.
"Nicotine extracted from tobacco (Nicotiana spp.) and other members of the nightshade (Solanaceae) family of plants has been used for centuries to kill sucking insects such as aphids, thrips and spider mites. This extract is highly toxic to virtually every living thing, however, including people, pets and beneficial insects. Nicotine sulfate is considered a botanical pesticide, allowed in organic gardening but is no longer available commercially because of its toxicity, but a family of synthetic chemicals that mimic the effects of nicotine on insect nervous systems feature in many insecticides." https://homeguides.sfgate.com/can-nicoti ... 82389.html
So you could also steep your tomato plant leaves. But again note that this would be harmful to beneficial insects as well as pests. If you are trying this, I would use it quite diluted and only on areas where you are having a lot of problems.
Another is chrysanthemum. Natural pyrethrum is derived from chrysanthemum flowers and is pretty environmentally safe (ie. for pets and people) and not persistent in the environment. But it is also harmful to bees and other beneficial insects. Pyrethrins and pyrethoids (such as permethrin) are highly concentrated synthetic versions that are used commercially. https://www.helpfulgardener.com/forum/vi ... 11&t=57653
A simple soapy water spray is effective against soft bodied insects like aphids.
Say more about what the problem is you are trying to deal with and we can give you more specific information.