Edited to add: I take it for granted that everyone knows the true "first step"... felt (wash it in a hot wash & throw it in the dryer) your sweater before you cut your mitten pieces. This will prevent your pieces from unraveling.
The first step is to create your pattern. I don't know how to make patterns to scale online, but hopefully you can create one for yourself, based on their shapes. It isn't too hard, considering that I made a pattern for my son's mittens the other day from eyeballing the adult pattern. I originally started with this pattern, but it was much bigger than what I wanted, so I tweaked it until I got my current pattern. Maybe that will work best for you too. If you eyeball it, the important thing to remember is to make sure your 2 palm pieces end up being a total length of about 1/2" longer than your back of hand piece, since you will be sewing the 2 pieces together, that will be your seam allowance. You will be starting with 4 pieces... the back of the hand, the 2 palm pieces, and the cuff. The cuff can be the cuffs of the sweater (what I used here), or a piece of ribbing from the sweater sewn into the same tube shape.

Ok, ready? Sew your 2 palm pieces together in the middle, wrong sides out, creating the thumb.
Now sew the back of hand piece to your palm piece, remembering wrong sides out. You will want to move the thumb out of the way as you go.

If you are making Mitten Doublers, that's it... you're done!
Making mittens? Now it's time for the cuff. Take your cuff & stick it inside with the cut edges lined up. When you look inside, you should see the "right " side without the seam. Now sew your cut edges together, stretching a little as you go.


Now you are done. Flip them right side out, roll up your cuff, & enjoy your cozy, eco friendly, cute, naturally insulated mittens!
At this point many people like to secure their cuffs with a button or decorative stitching, but I like to leave them as-is, so they can be rolled down & stuck in a coat sleeve when it's extra cold or snowy. The choice is yours!