Decide what you need most

What kind of terrain will you face?

What kind of climate are you going to deal with?

Urban or Wilderness?

Texas or Alaska? Not just these two states but you get where I'm going with this.

Winter or Summer?

Load your gear

Bottom, non urgent- the bottom of the pack is where you should put your sleeping bag, blanket, tent, or any other gear that won't be needed until camp is established and being set up. This is where your least used gear should go. Core, urgent- The core of your pack is the middle. This is where heavy items should go. It will keep the weight close to your back and should help maintain a good center of gravity. Cooking gear, food, clothing, anything that you want to get at in a decent amount of time but is not emergency gear will go in the core of the pack. Top and pockets, emergency- All of your essential gear should be at the top of the pack or in the external pockets. Things that you need immediate access to like snacks, flashlight, compass, maps, firekit, your knife, a pistol, or any other gear that will be needed without having to dig for it.

A few more important tips

Keep it organized. You could even get different colored stuff sacks and color code your gear. Just make sure you know where everything is.

Two is one and one is none. Make sure you have a backup for everything. This is where multi purpose gear comes in handy. Don't have exact duplicates though. For example if you have matches you should also pack a lighter and a ferro rod. This way there are three tools that basically all do the same thing but if one method isn't working you have other options. Always have a spare for the most important gear in your pack.

Fill all empty spaces. This is especially important with the core of the bag. You don't want heavy gear shifting around inside the bag while you walk because it'll screw up your center of gravity.

Pack based on weight and priority. Non urgent, Urgent, and Emergency gear

Don't just assemble a bug out bag and leave it sitting in a corner. Use it and get to know it very well.

Rotate the food, vitamins, and medications inside your pack. The last thing you want is to have everything you need only to realize that its been five years and all the stuff is expired.

Don't hang anything on the outside. This draws attention, makes noise, and can get caught on vegetation and branches along the trail.

A huge pack that looks like a survival powerhouse is going to attract attention and will probably get stolen. Keep it low key and blend in. If it looks like you are just an average joe you are less likely to attract attention.

Use compression straps. A pack comes with compression straps for a reason. They tighten the bag up to bring it close to your body. To really get an ida of how this principle works hold five pounds close to your body. Now hold it with an outstretched arm. Which was easier?

Make sure there's extra room. You're probably going to come across useful items along the way to your evac point. As long as there's room in your pack these items can be picked up and put in the pack instead of trying to carry them.

Knowledge doesn't weigh a thing. The more you know the less you need to carry. An example would be a fishing pole. If you can make a improvised pole you wont need to put a pole in your bag.

Shoot for lightweight and mobile as opposed to big and bulky with everything but the kitchen sink. Back injuries suck and so does having your gear stolen because you couldn't evade a thief. The emergency you packed for could put you in a without rule type society where people who didn't prepare will try to take advantage of those who did.

More info I found on other websites

Before we get into how to pack a bugout bag there are a few things I feel I should clear up first. If your pack weighs 80 pounds its useless. You do not need all that weight to survive for 72 hours. The whole point of a bug out bag is to get you out of an emergency situation quickly and keep you alive for three days. Professional backpackers say that your pack should be at most 25% of your body weight. That weight is for non emergency situations, if you have to evac that is way too much weight. Your pack should be as mobile and lightweight as possible so that you can still run, duck, crawl, climb, swim, all the things that will be needed to escape a dangerous situation.The title says it all, figure out what gear you absolutely need and ditch the rest. You are trying to get by for three days not a year. Before you can decide what to put in your bag you have to come up with a plan.There are a lot of questions and you can't possibly answer them all but these are things you need to figure out to get the best idea of what gear you will need. Establish an evac plan before you select your gear. Think about the challenges you will most likely face. Then after you have a plan make a list of gear you think you need and lay it all out on the floor. Separate the gear into piles based on urgency and intended uses. Eliminate the things you don't absolutely need. An example of this would be something like I have a Leatherman and a pair of needle nose pliers do I really need them both or would I be able to just take the Leatherman? Try to pick gear that has multiple uses. Duct tape for example has a bunch of uses. It can be used for cordage, repairs, or first aid. You don't want a heavy pack that is overloaded with junk. Once you've gotten your gear down to a bare minimum go through it one more time and take out anything else you don't absolutely need. In my opinion weight reduction is the most crucial part of loading a bag.Now that you have reduced the weight of the pack and organized it, test it out, Go for a few mile hike with the bag. After the hike camp out overnight with just the gear you have. This is the only way to know if you are actually ready. By using the gear in a non survival scenario you will find out what you need to add and what you can take out.