Joy, Pinching out means that some plants benefit from the tips of their stems being pinched out to induce the plant to become more bushy. Just nip out the topmost leaves on each stem or branch. Most annuals are treated this way at the early growth stage and I do it all the time with fuchsias,daisies, petunias, impatiens, vinca etc.
It's been a long time since I raised plants from seed so maybe I'm not equipped to answer your question - maybe someone else can come up with it. However, on young plants I usually pinch them out when they are about 5 inches tall. Hope someone can help you further Joy :-)