This is a super easy project. I'm a fairly beginner sewer. The pocket hand warmers were actually an afterthought as I was sorting through my scrap fabric deciding which to use for the hot pack. Hot packs are awesome. They're great for sore muscles. I often get lower back and knee aches. You can also use them for cold therapy by keeping them in the freezer.
Start by cutting your fabric to the desired size, keeping in mind seam allowance.
I did 7x12 for the hot pack, 3x4 for Finley's pocket warmers (she's 3) and 2.5x3.5 for Carson's (he's going on 15 months). For an adult, 3x4 would probably be perfect. I wouldn't go any bigger than that.
No worries about crooked cuts or loose threads. They're going to be flipped inside out.
Each pack needs two sheets of fabric. Stack those two pieces, pretty sides together, un-dyed sides facing out, and sew around three sides, leaving a smaller end open. Flip inside out.
Now comes the rice.
Fill the heat back 3/4 full.
Fill the hand warmers half way.
Add in 10-15 drops of your favorite essential oil/s and shake shake shake. I only did the oils in the hot pack, not the hand warmers.
Then, flip the cut edges in to make a clean edge and sew across to seal.