This is something ive been doing for quite some time now. i call it...... well i call it cool line flying around your screen that is sure to make to new grounds or eBaums world or what ever viral video/game site you submit it to.
Right under the time line you will see something that says 12 fps (default) just double click it and it will bring up a properties box. change that 12 to a 24 click ok and your ready.
Step 2: Setting up your timeline
Ok to the left of your time line is a box with your layers, all you need to do is make a new layer and rename it line 1. then rename the first one ground. lock the ground layer. under your time line to the left there are 5 little boxes, one will say onion skinning click that one not onion skinning out line.
Step 3: Almost ready
on your first frame right click on grounds first frame, where you drew your ground in the time line and hit copy frame. go to frame 500 or so on you time line, all depends on the intended lenth of your video and create a key frame(shortcut f7) on the ground layer, then right click and hit paste frames.
Step 4: Select you brush
if you want thing to look good u must u the right size brush i like the third on towards the top
Step 5: Lets draw
ok now make sure you zoomed in to 100% and draw you first like half on the canvas and half off (not pictured) to make it look like your line is flying in from the side. after the line is drawn hit f7 and start the next line halfway on top of the next one and keep going on and on and on as u can see onion skin lets u see the past few steps but it wont show up in your video.