Depending on where you live (I am sure there are non-US equivalents), the local county 4-H and/or state Department of Natural Resources might have pamphlets and even packets of seeds for local pollinators. I added several to my garden and got a lot of neat butterfly species to visit this year.
One thing I learned from my local 4-H is that monarch butterflies only eat milkweed--the linked Wikipedia article clarifies that they only eat from the milkweed genus--but that was a surprising fact. The species within that genus are also not equivalent in terms of function for the monarch's lifecycle, as the linked article also mentions, so it's important to plant an appropriate species with respect to your location.
In addition to milkweed, there many plants that butterflies (and other pollinators) enjoy--I've had great success with zinnias, coneflowers, yarrow, and a few others, too. So it is worth taking a look at the resources in your area and planting native pollinators when you can.
One thing I like to point out for people is that milkweed has the second-most advanced flowers on the planet, following only orchids. They come in a massive variety of colors and shapes, and usually multiple are available in each zone as a native choice, so you have a little flexibility.
Definitely seconding the idea that you need to plant something appropriate for the monarchs that migrate through YOUR area.
means 'complex' - from Wikipedia " Five petals reflex backwards revealing a gynostegium surrounded by a five-membrane corona. The corona is composed of a five-paired hood-and-horn structure with the hood acting as a sheath for the inner horn"
Most difficult concept to grasp is "stop mowing for some months and they will come".
People has been brain-wired for generations with the absurd idea that everybody must to play golf in their lawns, and spend the weekends mowing, or will be tagged as a lousy bad neighbor. Many of the species that were easy to found some decades ago vanished locally by this ignorant mentality.
But is so simple as letting a part of the lawn more natural and cut it three times a year; and avoid poisons at your home that don't solve anything. Introduce some wildflowers and people suddenly starts changing their view.
Assure to include a wild hedgerow somewhere in your property with a mix of native flowering and nurturing shrubs and; if you are interested in butterflies; place a moth trap near this shrubs. You will be surprised.
A while ago I ran across some video on the history of lawns. They started with the rich/royal in England(?), and it was essentially a flex. "I have so much money I can afford to have unproductive land and pay people just to make it look pretty." When I see people spending their free time working on their own lawn, it seems to defeat the original purpose. It's not a symbol of wealth and status, it's an obligation, and an expense, which is likely taking away from other areas of life. Not much different than someone sacrificing their savings and retirements to drive around a fancy car. I know some people enjoy working in their yard, but I have to believe they are in the minority.
I have to reply here. The problem is not the mowing of the lawn:
The problem is the lawn itself!
The thing to do about the lawn, if you want to do the right thing, is:
Clear off the grass!
Edit: This comment was way too long. Congrats to those of you who read it before I shortened it.
The crux of the matter is:
Grass will absolutely kill wildflower, so if you want a pretty flower carpet, clear off the grass before you sow. Remove it with roots, and by hand, as a machine will only chop it up and it will regenerate from there killing your flowers. Start with a small patch the first year. Also, pick any wildflower mix, even if cheap. Once the grass is gone it will grow nicely. You do not have to tend it. Just sow it, and leave it.
> Monarch
Please, the Monarch is not the only butterfly around. Besides, monoculture is NEVER good, not even a focus on one particular butterfly type. There are several, some visible by day, and even more (not very visible) at night.
What's more, there are beetles, bees, all kinds of weird flys and whatever kind of insects in absolutely fascinating colours and looks that seems taken right out of a LLM model, so weird it is. Sit by your patch daily after it comes up - even if just for 10-15 minutes - if you have curiosity, you will be amazed.
It depends on what you want to attract. Many caterpillars need tall grasses to eat and hide so you still want to allow some, but I agree with you about the basic ideas. Monarchs are only one species but not the only desirable species
In Mediterranean Europe you may want to attract the swallowtail so you need to plant fennel instead
It requires very little effort to get started and the rewards keep coming. If one starts with Monarchs, you have the help of voluminous amounts of research and advice online to help. The whole process is fascinating, and one will often start adding new species as familiarity with the first is gained. An added bonus is the chrysalis of a Monarch looks like a precious gem.
The wikipedia article seems quite US-centric, but for those of us in other parts of the world, there are likely to be resources available if you search, both online and in print.
My inspiration as a child was L. Hugh Newman's Create a Butterfly Garden; probably out of print these days, but it was a delight of a book for a youngster in the 70s.
One thing I learned from my local 4-H is that monarch butterflies only eat milkweed--the linked Wikipedia article clarifies that they only eat from the milkweed genus--but that was a surprising fact. The species within that genus are also not equivalent in terms of function for the monarch's lifecycle, as the linked article also mentions, so it's important to plant an appropriate species with respect to your location.
In addition to milkweed, there many plants that butterflies (and other pollinators) enjoy--I've had great success with zinnias, coneflowers, yarrow, and a few others, too. So it is worth taking a look at the resources in your area and planting native pollinators when you can.