I traveled to Nicaragua to a village where almost everything was primitive. They had a beautiful hand dug out canoe which was where I saw one of the few manufactured things they used- rope! Here they were in the jungle with so many cordage options and they purchased nylon rope. There are so many needs for rope and string, fishing line and bow strings. Learning to make cordage from natural materials including plant fibers and inner bark, is really a foundational skill. This page includes articles and videos on various techniques and harvesting. You can practice with almost anything- I've found corn husks make great cordage and can be a fun family picnic activity!
Retting bark to make cordage: Walt Gigandet Making cordage is one thing. Knowing how to prepare natural materials to do so, is another. This is a great video on harvesting and preparing bark for cordage.
Make Great Cordage out of Grass: Nepheron This is a very succinct video that clearly shows the process of making cordage and splicing. The only change I'd recommend is just bending a single bundle in half (off a bit so splices are offset) instead of the knot he shows at the beginning. Depending on your use, either way works fine.
Handbook of Knots and Splices: Charles Gibson: The complete book which includes information on ropes, splicing and a variety of knots which include diagrams.