I tied a simple knot in the center of the ribbon and then two knots on either side. I tied the knots so that the tail headed in the direction I needed it to to create the top of the heart shape.
Note that the knots are mirror images, reinforcing the symmetry of the heart shape. If you find the ribbon doesn't want to stay in the knot, use a glue dot to hold it in place. You do not want to pull the ribbon tight!
After adhereing the ribbon knot to the card panel, I tied the bottom knot on one side and cut the ribbon. The pieces are glued together with the ends tucked out of sight.
This little step -by-step tutorial should help:
I used some of the red Diagonal Stripe ribbon to show you how easy it is to tie a knot and turn the ribbon in the shape you want. The secret is not to pull the knot tight, but to allow it to lie flat.
The red diagonal stripe is easy to keep flat - with some ribbons you might want to use a glue dot to hold the knots in place while you are working. Depending on the ribbon, 5 to 6 knots will bring you full circle. I cut the ribbon and glue the end under a knot.
Here is a card where this technique is used to make a holiday wreath. Instead of cutting the ends, I tied a bow in place of the 6th knot.
With this wreath there is a lot of space between knots. On tag 18 of my Snowflake & Ribbon Advent Calendar, you can see a wreath made from 5 knots tied one after the other with no space in between.
Double faced ribbons are best for knotting, because the ribbon will flip front front to back as you tie knots as you can see here when I used the Chevron Grosgrain Wisteria ribbon from the December Monthly Ribbon Club.
You may want the look of alternating color and pattern, so I encourage you to play around a bit.