To make teardrops, start with your bag at a 45 degree angle to the paper, tip slightly off the surface of the practice sheet (click here for printable practice sheet
- 40kb .pdf). Start squeezing at a 5 on the pressure scale until the round side of the teardrop is the size you want. Let up on the pressure as you pull back towards the tip of the drop, going from the 5 you started with gradually down to zero at the tip. Pull your bag away completely, and you're done with your teardrop. Make a row or two of these until you're comfortable.
Even if you want the tip of the teardrop pointing up, you will always make them pointed-side down like you just learned. Cookies are very mobile, so all you need to do is turn your cookie upside down to make your tear drops and then turn it back around to continue decorating.
Now that you have that down, try a row each of the slanting teardrops. You can do it by either angling your bag appropriately, or tilt your practice sheet to the right or left as needed. Try it both ways and see which way you like best.
Hearts are just two slanted teardrops that have their tails meeting in the same spot. Practice making hearts by first making a right-pointing teardrop and then a left-pointing teardrop right next to it.