That's the part that takes the longest to master. Give it a try each time you shave, but don't keep going over the same area again and again. If you can't get it to work, make note of what you think might improve it next time, then finish up with your DE / cartridge. No need to punish yourself while you're learning.
It also helps me to wet the chin with fresh lather right before shaving it. Then small, light strokes.