How to go about acquiring a startup?

If the other business is smart, they won't let you acquire them at all and will force you into a B2B pricing package. Obviously you demand their product and are willing to pay a high fee; sounds like a perfect time to bait you into a contract.

Unless you buy them out for a ridiculously eye-watering price, I don't see how this could work well. At founder-scale you should be focusing less on mergers & acquisitions and more on building your product. A good place to start would be engineering a contingency plan if you can't acquire said company.

Do some research (maybe you already have), and then just reach out to their founder/CEO directly. It can't hurt to have a conversation.

If they are pre-seed, what do they have built already and how do you know about it? What stage is your company?

there is no fixed path, start with email and then follow up if they respond.

why raise the cost by using a middle men?

seems like you aren't really ready to do this from your questions.