There are a few ways you can do this. It just depends on exactly what you're selling.
Items with fixed options
If you have an item that has up to two fixed options, such as color or size, add variations to your listing.
That way, buyers choose what options they want while checking out.
If you offer personalization, such as initials engraved on a necklace, let your buyers know what you provide in the item description.
One of your listing photos could be an example, something you made in the past and can make again to the buyer's specifications.
Ask your buyers to let you know what they want in the Add an optional note to seller box they'll see while checking out.
Some sellers do both. They offer variations and ask buyers to leave additional specifications in the note box.
Private and custom listings
If you'd like to create private, custom listings based on buyer requests, you can enable the Request Custom Order tool.
Also, here is a Seller Handbook post with some tips on managing and using the custom order tool.