Hello and welcome from the uk!
Have a look round the forum for good designs- My 4th sling was my first one that actually worked, but I've made 30-40 since that are generally improved from that stage!
If you lack equipment you can make a reasonably effective braided split pouch with 100% just twine. Options increase if you get leather, a hole punch, sewing equipment etc
Tutorial for basic split pouch:
http://www.slinging.org/articleimages/20/19.htmNote: I'd recommend 5-6m for a decent length sling not 3m. Maybe it's a matter of how tight you braid but I'd struggle to reach 20'' with 3m cord, which is very short for a sling.