Step 1: Prepare the strawberries by washing them gently, hulling, and quartering. For larger berries, consider cutting them into smaller, uniform pieces.
Step 2: Prepare the oranges by washing them thoroughly. Thinly slice the oranges (including peel and pith) using a sharp knife or mandoline, removing any seeds. Alternatively, zest and segment the oranges, finely chopping the peel separately.
Step 3: In a large, heavy-bottomed pot or Dutch oven, combine the prepared strawberries, sliced oranges (or chopped peel and segments), and granulated sugar. Stir gently to coat all the fruit.
Step 4: Let the mixture sit at room temperature for 30 minutes to an hour, allowing the fruit to release its juices and the sugar to begin dissolving (this is called maceration).
Step 5: Place the pot over medium-low heat, stirring constantly until all the sugar is completely dissolved and no grittiness remains.
Step 6: Increase the heat to medium-high and bring the mixture to a full, rolling boil that cannot be stirred down.
Step 7: Reduce the heat to medium-low and continue to simmer, stirring occasionally (especially as it thickens), until the mixture thickens considerably and reaches the gelling point. This can take 30 minutes to an hour or more.
Step 8: To test for set, place a small amount of hot marmalade onto a frozen plate. Let it cool for 1 minute, then push the edge with your finger. If it wrinkles and forms a skin, it's ready. A candy thermometer can also indicate readiness at approximately 220°F (104°C). Repeat the test if needed with a fresh cold plate.
Step 9: While the marmalade is simmering, sterilize your canning jars and lids by boiling them in water for 10 minutes or running them through a hot cycle in your dishwasher. Keep them hot until ready to fill.
Step 10: Carefully ladle the hot marmalade into the sterilized, hot jars, leaving about 1/4 inch of headspace from the rim. Use a non-metallic utensil (like a clean knife or skewer) to gently remove any air bubbles.
Step 11: Wipe the rims of the jars clean with a damp cloth. Place the sterilized lids on top, and then screw on the bands until they are finger-tight (do not overtighten).
Step 12: For shelf-stable marmalade, process the filled jars in a boiling water bath for 10 minutes (adjust processing time for higher altitudes). Carefully remove the jars and let them cool undisturbed on a towel on your countertop for 24 hours. After 24 hours, check all seals; the lid should be concave and not flex when pressed. Store any unsealed jars in the refrigerator and consume within a few weeks.