Homemade weed killers generally work best around a garden (for example, the area along a garden fence, where weeds sprout up) rather than on lawns because they kill whatever they come into contact with (such weed killers are said to be “non-selective”). So if you spray them on a lawn weed, and if some of the spray misses the weed and gets onto your grass, the grass will die.

Vinegar
Vinegar spray is one of the best homemade weed killers. It's easy to prepare and effective against a variety of weeds. All you need for ingredients are:

A type of vinegar high in acetic acid
A surfactant (this substance doesn't kill weeds; its role is to help the vinegar herbicide penetrate into the weeds' leaves)

Pour the ingredients into a spray bottle and shake to mix.

Apply when the weather report says you'll be getting a few continuous days of sunshine. Rain would wash the vinegar off the weeds too soon and most of the damage happens when the sun hits the weeds' leaves.

Spray directly onto the weeds, being careful to keep it away from other plants.

Rubbing Alcohol Solution
This astringent substance works by removing moisture from the weed's foliage. Deprived of its life force, the weed dies. Mix the solution in a spray bottle for ease of use.

Preparation is as simple as diluting the rubbing alcohol with water: use a quart of water to dilute every two tablespoons of rubbing alcohol.

As with vinegar, apply on a sunny day, and spray directly onto the weeds, being careful to keep it away from other plants.

Boiling Water
This homemade weed killer is the easiest of all to prepare and use. You won't be doing any mixing at all, and you can apply it at absolutely any time you wish: day or night, with or without sunshine, even in the pouring rain.

Simply boil some water and pour it on the leaves of the targeted weed. Pour enough so that it seeps down into the roots, too. It's helpful to have a kettle to boil the water in, so that, when you pour, the stream of boiling water can be directed with a spout. But where precision isn't necessary, even a pot serves the purpose. Sometimes, though, your pouring must be precise, as when you're killing a weed that's growing right near a good plant: Scalding the good plant with the boiling water could harm it.