Reverse applique is very easy depending upon the design you wish to use. I wanted to share with you how I take a design from the fabric I'm working with, enlarge it & create a cute accent to whatever it is I'm making. For this tutorial I'm making an A-line skirt from a thrift store sheet & adding some 2nd hand fabric as my "peek-a-boo" fabric for the reverse applique. Let's begin!

Step 1: Step 1: Create Design

Something simple like a circle or triangle, using a simple design/stencil from the fabric you're using, whatever! The possibilities are never-ending :) You just pick out a design to cut out of one piece of fabric to let another fabric peek through.
I'm using an old school method of enlarging a design from the main fabric of my skirt. I'll enlarge it once more to the actual size I want (also because I'm actually making 3 skirts, I needed to copy the image 3 times). This will help me to asses what size my "peek-a-boo" fabric should be. You'll want at least 5/8in. allowance for the "P-A-B" fabric. I wanted to keep my "P-A-B" fabric tidy so I decided a simple square with hemmed edges would work best.

Step 2: Step 2 - Pin & Sew

Once you've decided where your design will go you'll need to pin the "P-A-B" fabric to the back & design outline to the front. Make sure the "P-A-B" fabric's right side is pinned to the back of the main fabric, so the right side can pop out once finished!
Now, I don't have a fancy-schmancy sewing machine. Not one that you can simply upload an image, press a button & voila! It's following the outline of the image. Nope, I have to do that manually... that's why I pin my design to the top, so I can follow the outline. Start. Stop. Twist, turn, adjust. Start. Stop. Turn. Start. Stop... this is the lesson in patience. Be patient with yourself, your machine and your project. Understand that the more complex the design, without the fancy-schmancy sewing machine, the longer & more tedious it may be. But then you can also revel in how awesome it turned out AND THAT YOU DID IT YOURSELF!! :D
Ok now back to the project...

Step 3: Step 3 - Removing Design Assistant

Now you just spent all that time tediously following this complex & awesome design, you don't want to rip out that paper & risk damaging all those stitches! So? I take a spray bottle & thoroughly moisten the paper. Yay! Now you can easily tear out the design w/out stressing your stitches.

Step 4: Step 4 - Cutting Out Design

Cut out design, leaving 1/8 to 2/8in from the stitches of the *MAIN FABRIC* ONLY!! <-- This is sooooooo important!! I use a simple "pinch & pull" technique; lightly pinch the top fabric with one hand & the bottom fabric with the other, then gently pull apart, securing that the top fabric is now away from the bottom & cutting can proceed without issue.
Leaving that 1/8 to 2/8in is also key. This allows for a bit of fray but not so much that it would endanger your design.

Step 5: Step 5 - Wash & Enjoy!

Now that the design has been cut out, you'll want to toss your creation in the wash... I use a regular wash cycle & 1 complete dry cycle *1 TIME ONLY* to allow for that bit of fraying. After that I use a delicate cycle & only dry for about 10 min, allowing it to air dry after.
Alrighty! Go out & sport your cute ____ (<-insert project here) with reverse applique!