Wash the makrut limes well. Cut them in half across their equator, pluck out the seeds, and cut them into thin half moons (gathering up as much of the juice as possible as you do this). Pour everything into a large bowl or jar.
Cut away the peel from the conventional and section out the lime flesh (this process is called supreming and there are lots of tutorials available on the internet if you need some guidance). Add the lime segments to the bowl or jar.
Cover the prepared fruit with four cups of filtered water and refrigerate for up to 48 hours.
When you're ready to make the marmalade, pour the fruit and water out into a low, wide pan. Add the sugar, stir it in and bring it to a boil. You can either finish the cooking process now, or cool it down and let it rest (as described in the blog post above).
Cook, stirring regularly, until the syrup thickens, and an instant read thermometer displays a temperature of 222°F.
Remove marmalade from heat, funnel into prepared jars, wipe rims, apply lids and rings, and processing in a boiling water bath canner for ten minutes.
When time is up, remove the jars from the canner and let them cool on a folded kitchen towel.