There are two ways to fill in a medium-large space (note: If it is a very large space to fill, I recommend glazing the cookie). The first is to use a large straight tip (#4 or larger) and fill in using the pressure control that I described with the teardrops. Start at the top and squeeze, varying the pressure depending on the space to be filled (more pressure for wide areas, less pressure as you go through narrow areas.)
The second method is to use steady pressure (#3 or #4 on the pressure scale, and zigzag back and forth, with lines touching, until the space is filled.
Practice each of these methods on your practice sheet (40kb .pdf) a few times until you are comfortable with them.