Trees, plants and furs may all be used to make, patch or supplement existing clothing. Dry leaves can be stuffed into your clothing to create dead air space, which will help you to be warmer. Have a look on the Survival Tips & Tricks page for more information.
For animal hides there is what is called brain tanning, where the brain of the animal is mixed with water and then spread on the non-fur side of the skin.
Preparation is required beforehand in that the skin must be stretched out and any flesh removed by scraping. Have a look at the links below and the video to the left for more information on the subject of brain tanning.
Here are some links to help you learn more about primitive clothing and brain tanning.
1. Wildwood Survival - clothing
2. Brain Tanning Furs by George Michaud
Have a look at the Videos page as well for more information on primitive clothing and brain tanning.