Okay, I'm going to assume you want to piece together two hoodies to make a super tall one, as it seems to be popular (personally I don't like it, but whatever). As for making one from complete stratch, you're on your own.
You need: Two (or more) hoodies, sewing machine, and scissors. That's about it.
1. Decide where the new part of the hoodie will go on the old hoodie, whether it will be a two or three piece design, etc, and cut it out accordingly.
2. Time to sew it. It looks better if you flip both pieces of fabric inside out, put them together so on both sides the inside is showing, the sew along the edge. This will create a smooth line when worn normally. Make sure to overstitch when you're done to prevent pulling apart. Finish it up, and hopefully, you should get something wearable.
3. Wear it.