If you are open to accepting customised orders, you need to state somewhere (e.g. in your profile or at the bottom of your listings) that you are happy to accept custom orders and that buyers should contact you through the Folksy messaging system if they have something in mind.
If you accept a custom order where a buyer has found and contacted you through Folksy, you must list the item for the buyer to purchase on Folksy. If you complete the transaction off-site, this is classed as fee avoidance and can lead to the suspension of your Folksy account.
To create a custom listing, add a new listing as normal with example images, if appropriate. Remember to state in the listing title and/or description that the item is a custom order for [buyer name] so that nobody else buys it. Make sure you set an appropriate shipping time and tick the 'custom order' box in the Returns Policy section. You can then list the item and email the link to the listing page to your buyer. Once the buyer has completed the order and you have received payment, you can start making the order!
Alternatively, if you offer a simple personalised version of an existing product, you can list the item with example images and state that the buyer needs to include personalisation details in the note to shop at checkout. Search Folksy for 'personalised' for examples of how other sellers do it!