I'd always admired throw pillows and the whimsy they bring to my living space; so much that I make my own.
I've used this technique time and time again and it's always worked and it only takes me no more than 20-30mins to complete.
Here you have a complete tutorial on how to make these removable covers.
I made my own custom pattern which in all honesty is a long rectangle.
My pillows at home are 18" square.
So you should +1" to your width side which means cut a sheet of craft paper at 19"Wx42"L for the pillow pattern.
What you need:
Step 1: Cut and Press Your Fabric
After you've cut your pattern from your fabric lightly press your fabric to remove wrinkles and creases. Use a steam setting if your iron comes with one to get the wrinkles out quickly.
Step 2: Fold Your Short Side
Begin by placing your fabric face down an then fold the short sides (19") over a 0.5" at the raw edges; then once more another 0.5". This totally incases your raw edges.
Step 3: Sew Short Side
Use a straight stitch on your machine to sew each edge. I've also used a twin needle just for extra flare.
Step 4: Mark Short Sides
Turn your fabric right side up; bring in one short side. Mark the top of that side in by 3" from the short side you just stitched (indicated by yellow) then bring in the other side to overlap it at the 3" mark. Do this for the other side too. Pin all sides to secure.
Step 5: Stitch Remaining Seams
Now that you've stitched the short sides stitch the last tow seams. You can use a serger or a sewing machine. I used my serger. Make sure when sewing to capture all the thickness. Look how cleanly the serger worked here.
Step 6: Turn Inside Out
Now turn your cover right side out. If your a perfectionist; poke all four corners out an then press lightly with a steam/regular iron.
Step 7: Slide Pillow Into Cover
Lift one side of the opening to insert your pillow.. Then stuff the other end of the pillow into your cover. As you can see the entirety of the pillow is incased an covered.
Step 8: Done
Step back an marvel at your job well done