First up, we'll start with sketch. I'd suggest new artists to put in effort and take your time in this procedure, keep your work clean and tidy to ease up your inking and erasing procedure later. Below is a sample of a sketch in progress, I've darken the lines so that you can have better view of the manga drawing process.
I start it off by setting up the borderlines, there are two borderlines in manga usually, the inner frame is where all your drawings and dialogues should end, especially dialogues, try not to go beyond this border.
But let's say you have some awesome image of your hero slicing up the villain monster that you want to show, what you can do is to extend your drawings and ONLY YOUR DRAWINGS to the outer frame. Nothing goes beyond this borderline because that is the bleed that your printing house will need.
Roughly sketch out the conversation bubbles and allocate your characters or backgrounds to where you want to show them.
Now sketch your manga.