Clean, chill and finely slice the clementines.
Combine the clementines, sugar, and lemon juice in a heavy-bottomed pan. Bring the mixture to a simmer, then remove it from the heat, put it in a large bowl and place it, covered, into the fridge overnight.
The next day, add about 1/4 cup of water (this isn't in the original recipe, but I like to do it), simmer the mixture again, and let it sit another night in the fridge.
(Some reputable sources say this step isn't necessary and that you can go ahead and finish the marmalade on the second day, but I've continued to let it sit for a second night -- because it works, or maybe because I'm lazy, not sure which.).
The following day (this is day two or three, depending on whether you include Step 3), put the mixture back into the heavy pot and add:.
apple jelly kaffir lime syrup slivered almonds
Bring the mixture to a boil, stirring gently. Skim if necessary. Boil at high temperature until the mixture reaches the jelling point (220F) -- about 5-10 minutes.
Remove the mixture from the heat and stir in the almond extract.
Ladle the hot mixture into sterilized half-pint jars and process. I left 1/4 inch head space and processed for ten minutes in a water bath canner.