Use old wax to create another candle from prior candles.

Step 1: Gather Supplies and Prepare Wax

Old candles, any type.
A microwave safe glass jar.
Some wicks.
Scrape the wax out of the jars.
Break up the wax.
Remove any metal wick holders or other debris.

Step 2: Fill Candle and Cook

Start filling your jar and put in the wick.
This is the tricky part.
Fill the rest of the jar while keeping the wick in the exact center.
Put the filled jar on a microwave safe plate and nuke it for a minute or two at a time.
Watch it while it melts.
Don't disturb the wick.
Push down the wax then top off with more old wax after each cooking session.
Microwave cooking time are approximate.
Cooking times may vary depending on the wattage of your microwave oven.

Step 3: Done

Let your candle cool all day.
Trim your wick and light it.

The jar used here was a pine candle and the outcome was a mostly green candle.
The smells were waxy with some pine and sweetness from other source candles that I collected.
My plan was not to do this in the microwave, but alas it was easier.