I've had a few requests for a manga drawing tutorial lately, so here's a step-by-step method that I use to plan each page.
First, you write the script. Just write down the dialogue and actions at first. It also helps to go through and break the script up into panels. I also tend to put in emotions and any other details I think might help later on. This is an example I put together for this tutorial (I'm not going to start writing "Goten's birthday party!"):
1 Goten: flies to Capsule Corp, happy
Next you sketch the layout. This is NOT the final layout, just the plan it only needs to be about a third of the size of an A4 page. A good key to go by is that important panels should be LARGE. The two most important panels are 1 (because it sets the scene) and 5 (because it's a turning point). The middle three panels are transitional, and don't need emphasis.
I draw Japanese style, from right to left, so when reading the page, your eyes tend to follow this path. To make your page flow well and be easier to read, you should make your action follow it.
And now we're ready to fill in the panels. It doesn't need to be neat (generally it only takes me a few minutes); the amount of detail is up to you, but the more detail you do now the easier it will be to draw the actual page later. Even a plan with stick figures is better than no plan at all. If there's anything not obvious in the plan, or than you want to change in the final page, then write it down.