Pick a central spot on the page from where all the lines are meant to converge or draw out. This is almost always the very center of the page, which is why centering the character helps if the picture is of a single character.
Plot out a circle in the center of the page where the lines will stop. This should encircle the character's face or whatever is the center of attention. If you must actually draw the circle, use a blue pencil as it won't appear in copies.
Draw the first lines at equal intervals along the page. Imagine splitting the panel into a "pie" of eight pieces and draw the lines at the dividing spots. Make these lines thick and dark.
Add more lines in between the main lines you just created. Make these lines lighter and shorter than the first ones, stopping them before they reach your "encircled area." It helps to make the ends of these lines form "arcs" around the encircled area.
Open up your anime art program and open the file that contains the panel you want.
Go to the "layer properties" window in the program and select the "focus lines" tab.
Drag the encircled area that appears on the screen and place it in the area where you want the lines to converge. This special cursor usually has an "X" or something similar in the very center.
Input the needed information for the lines in the boxes. This includes the length and width of the lines, the angle of the lines, any curve you want to them and how far the lines will be from the center.
Click the "Ok" button to have the program post your lines on the panel.