If you cannot take her to the vet, first step is to remove the string, if you haven't already. You will have to dig for it, and get all of it. This is likely going to need a helper to hold the chick. Have supplies ready. You'll need betadine to clean the wound, styptic powder such as Quik Stop, to stop bleeding, and Neosporin (without pain ingredients) to apply to the wound. Bandages, non stick telfa pads and vet wrap to keep it clean afterwards. !/2 a baby aspirin for pain would be good. Do not try to amputate the toe. It may fall off on it's own, but the most important thing is to get the string out.
Clean the toe well first with betadine, use a sharp pointy scissors (disinfected first by soaking in alcohol) to dig down to the string and cut it. You may have to pull off some scab to get to the string. Once you get the string out, rinse it with betadine and put pressure on it to stop bleeding. If that doesn't stop bleeding, then apply styptic powder. Then apply neosporin and bandage with non stick pad and vet wrap to hold that on. Just wrap the whole foot. Keep the chick warm and inside to keep the foot clean. A drop or 2 of nutridrench before and after may help boost the chick. Change the bandage and clean the wound daily. Antibiotics would be good if you can get them, we can't, here ,w/o a prescription so I don't know if you can.
The chick may already be septic, and this will be hard on it, so prepare yourself. But without getting the string out, she has little chance. However, if she survives, even if she loses the toe, she should have fine quality of life. It's worth trying, IMO.