The first question you need to ask yourself is, do I absolutely need to move this patient. If somebody is too injured to walk out themselves, you want to avoid moving them without proper equipment if at all possible. If there's any chance of a properly equipped rescue team arriving in the same amount of time (or even more time than) it would take you to get out yourself, it's probably best to have the patient stay put.
Moving a patient with an improvised stretcher obviously puts them at risk due to dropping, which could exacerbate any injuries they have, especially if they have any spinal injuries. Worse yet, it also puts the rest of the group at risk – carrying a stretcher is difficult and tiring on even terrain, let alone on steep and rocky trails. Other group members could easily twist an ankle, break a leg, etc.
What I'm getting at, is that before you start dragging people around on makeshift stretchers, stop, sit down, eat a snack, drink some water, and thoroughly analyze your situation and what rescue options might be available. You're likely a couple days from definitive care anyways, so an extra hour is better spent coming up with a good evacuation plan than getting them to the hospital a little bit faster at the risk of never arriving at all.
In pretty much any first-world country (and many developing nations too), if you can get word out to a rescue team, they'll be better equipped to help than anything you can rig up yourself, no matter how remote your location. A stokes litter and extra people to switch out carrying it go a long way. If you're really in an isolated location, a helicopter with a winch can extract you from almost anywhere. Don't expect to move more than a mile per hour at best when carrying a stretcher. Is it really faster to carry the patient out than to send a couple people out with a note to fetch professional help? If you can hike out and get help in one day, that distance means it'll take you probably two days to a week to carry them out, if you make it at all.
All that being said, suppose you really don't have any other option but to carry your buddy out. After attending to any immediate threats to life (airway, breathing, circulation, serious deformities, exposure to environment), every person in the group needs to sit down, eat some food, drink some water, and generally make sure they are as well-taken care of as possible before carrying on. You're going to be carrying 100-300 pounds probably for several days, plus all your own gear. Making sure every group member has their own needs taken care of ensures that they'll be in good enough condition to then take care of the injured group member. Don't let your rescuers become rescuees themselves.
Now that everybody in your group is in good condition, you can start building a stretcher. Other websites have pretty detailed instructions for different methods depending on what equipment you have available. Here's a good one: Improvised stretcher: UK Real First Aid
The general idea is, you don't necessarily want to make a setup with two rigid poles and something in between. Instead, you want to bundle your patient up like a burrito, provide some rigidity, and have a package you can pick up or even drag if necessary.
If you're in an even mildly cold environment, it helps to put the patient in a sleeping bag first. They aren't working, so they'll get cold quick. Regardless of environment, a camping pad is a good first layer. You can then put trekking poles, ski poles, or even use the patient's backpack frame (since they won't be carrying it now) to provide some structure. Lash the whole thing together with straps (again, consider using the patient's backpack as a starting point), utility cord, etc. A sleeping pad and 50' of nylon utility cord should be standard on any trip, especially in such a remote place that any of this would be necessary.
Once you've made your patient burrito, use as many people as possible to lift it. If there's any concern for a spinal injury and you have enough people available, have one person solely dedicated to immobilizing the patient's head. Transporting by an improvised stretcher will cause them to be jostled a lot, and you need to have somebody hold onto the sides of their head to prevent damage. If not enough people are available, you can use jackets on either side of their head to immobilize, or a SAM splint if one is available.
When you're ready to lift, the person holding the head is in charge. If they're not present, then choose a person to be in charge. That person should verify individually with each lifting person that they are ready, then ask the entire group if anybody is not ready. Do a count down before lifting, then lift slowly and together.
Once you're carrying, it's imperative to walk slowly and carefully to avoid dropping the patient or injuring any of the carriers. Stop frequently to rest. A slow evacuation is more likely to be successful than a rushed evacuation. When setting the patient down, follow the same procedure as picking them up: head holder is leader, check with the group, count down.
It may be safer in some places to drag the patient than to carry them. If going through very narrow or unstable terrain, you don't want to drop the patient. While being dragged is uncomfortable, being dropped is much worse.
Finally, remember to stop often and keep the entire group warm, hydrated, and fed. Hungry, tired, dehydrated, or cold rescuers will quickly become patients themselves, exponentially worsening your situation. Also, don't forget to continue monitoring the patient's vital signs throughout your journey. It's easy to get so focused on the evacuation that you forget to pay attention to the patient you're evacuating. Keep a log of their heart rate, respiratory rate, skin color and temperature, level of consciousness, and any reported pain throughout your journey. This is very helpful to monitor trends and see if their condition is stable, improving, or declining.