Introduce your main characters and themes in the first third of your novel. If you are writing a plot-driven genre novel make sure all your major themes/plot elements are introduced in the first third, which you can call the introduction. Develop your themes and characters in your second third, the development. Resolve your themes, mysteries and so on in the final third, the resolution.
Don’t panic. Midway through writing a novel, I have regularly experienced moments of bowel-curdling terror, as I contemplate the drivel on the screen before me and see beyond it, in quick succession, the derisive reviews, the friends’ embarrassment, the failing career, the dwindling income, the repossessed house, the divorce . . . Working doggedly on through crises like these, however, has always got me there in the end. Leaving the desk for a while can help. Talking the problem through can help me recall what I was trying to achieve before I got stuck. Going for a long walk almost always gets me thinking about my manuscript in a slightly new way. And if all else fails, there’s prayer. St Francis de Sales, the patron saint of writers, has often helped me out in a crisis. If you want to spread your net more widely, you could try appealing to Calliope, the muse of epic poetry, too.
The main rule of writing is that if you do it with enough assurance and confidence, you’re allowed to do whatever you like. (That may be a rule for life as well as for writing. But it’s definitely true for writing.) So write your story as it needs to be written. Write it honestly, and tell it as best you can. I’m not sure that there are any other rules. Not ones that matter.