Intarsia Crochet Outlining (also called Tambor Crochet) is a finishing step that sets your work apart from all the rest, and gives you the ability to decorate if you choose. Your work takes on the appearance of a separate design that's set into the crochet piece.
It also brings your work more to the level of original Intarsia, and further away from just trying to put a shape into your crochet project.
With these two lessons (Intarsia Crochet Basic can be found here.) you will be able to use both ways, depending upon the finished look you desire.
Intarsia Crochet Outlining uses the Top Stitch (also called Tambor Crochet) making the work very fast and easy. If you don't yet know how to Top Stitch the page offers complete instruction and you can even get a copy of the page for yourself.
The best way to share this with you, and teach this skill, is to share exactly what I did to the heart above.
I went around the outside of my heart twice. The first round I did toward the outside of the heart, in white. The second round (you can hardly see it) I did close to the first, with blue. This totally covers the stitches that I changed color in, making the heart look more 'set in', not crocheted in. The inside heart is simply top stitched in, about one stitch from the outline.
I started at the bottom, and worked to the center of the heart top, with all my top stitching (you can just go around if you like). Below the heart, at the point, find the center most stitch. Pull your yarn around this stitch so you can tie it in the back.
NOTE: You may want to leave long yarn ends so you can weave them in (I didn't - my big mistake).
Draw up a loop, holding your working yarn in the back (this is better explained on the Top Stitch page - if you don't understand please have a look there).
Move your hook one stitch up the side of the heart, draw up a second loop, and slip stitch it into the loop that was on your hook. Continue doing this until you reach the point you wish to stop.
KEEP YOUR STITCHES SLIGHTLY LOOSE
Trace around the outside of the heart, just where your stitches have changed color. At the end pull your loose yarn to the back. I started at the bottom again for the other side, but you could keep going here if you want to.
The picture below is overexposed to 'try' and show you the blue row next to the white. By doing one more round, in the color of the design, you cover (much better) the crochet stitches you did to change color. This also gives the 'set in' appearance that Intarsia Crochet Outlining is designed to create.
The white on the inside is just freehand following the outer stitches. You can see how Intarsia Crochet Outlining really does change the look.