Slice and peel the mangoes: the mango has a fibrous seed running through the middle of it, lengthwise. Slice the mango flesh away from one side of the seed and then the other. Discard the seed.
Remove the skin from the mangoes: Use a tall sturdy drinking glass, and hold the bottom edge of the mango half on the lip of the glass. Push the mango down so that the lip of the glass slides between the fruit and the skin. Push the mango all the way down until the fruit slips out of the skin. Repeat with the other mango half. Discard the skins.
Chop up the mango into ½-inch pieces.
Cook the ingredients:
Place the chopped mango in a large saucepan, and add in the lemon slice, orange juice, pectin and the sugar.
Bring the ingredients to a simmer over medium heat, stirring often.
When the jam reaches a simmer reduce the heat slightly. Simmer the mixture for 5-6 minutes. While it cooks, use a potato masher to gently mash down the mango into pulp (be careful not to splash yourself.)
When the jam is thick remove the pan from the heat.
Allow the jam to cool a bit, then transfer it to a heat proof jar. Allow it to cool completely, then refrigerate the jam for up to two weeks.