Step 1: Getting the Layers in Place
This is a good project that i recommend just straight copying if you're interested. But it's more just an example of some easy sewing, using things you probably already have. When you read the word "thermal" or "hoodie" here, take them as metaphor for whatever layers you're using, for whatever it is you're trying to do with your clothes.
I don't have any Before photos of this. But i doubt that matters because this isn't very tricky.
-First i put the flannel on inside-out, then put the hoodie on over it, right side out. Make sure they're both on neat and evenly.
-Put safety pins on either side of the zipper, pinning the shirts together
-Pin the cuffs of the sleeves together, being careful not to get it twisted
-Holding the shrits in place, pin the back collars together at the middle. Just one pin will do here, since the collar is also pinned in front at the zipper
-Unzip the hoodie, but leave the shirts on and pinned
-Cut the thermal down the front middle to match the hoodie zipper. Take both shirts off and keep them pinned together
Step 2: Sewing the Layers Together
Now that you can reach the collars better, it's important that they're centered when you start sewing them. Mark the back center point on both the flannel and hoodie collar, and be sure your pin lines up. Just keep doing that, halving the sections of the collars and pinning them together at the center points, until you have a pin like every 2-3 inches around the collar.
Sew it in place. I like using this Frankensten Stitch because it accommodates strech and reduces fray. Sew around the collar, alongside the zippers, and around the cuffs of the sleeve.
Don't sew it at the bottom. At least i didn't with this one here; i thought it would alter the shape of the hoodie so i didn't try it.
Sewing around the sleeve was like a combination of 1 and 2: the seam gets sliced so that the narrower sleeve can be sewn to the wider sleeve without warping it. I sewed around the cuff but not up the slice, because i thought i'd be glad to have the extra movement
Step 3: All Done
I turned this inside out to give a better view of how everything matches up, also illustrating the point that this could be a reversible garment! Like, maybe one side could be a business suit, then you turn it inside out and there's a business-casual suit! You'd be ready for anything- what a blast.