This is a very basic method of casting on which I recommend to beginners. It’s a good all-rounder that will serve you well if you don’t know any other methods.
1. Start by making a slip knot, and placing it onto one of your needles.
Make sure not to tighten it too much. (If you’re having issues with slip knots, here is my basic method for teaching how to do one.)
2. Create a loop with the working yarn, which is the yarn attached to the ball.
Make sure that the working yarn lies on top of the yarn connected to the needle when making the loop.
3. Turn the loop so that the “front” of it faces the needle, and slip it on.
The front is where the working yarn crosses over the yarn attached to the needle. This is the important part which makes it all work.
4. Pull the working yarn to tighten the new stitch into place.
Remember, don’t pull it too tightly! You’ll need to work these stitches later, and that will be hard if you can’t get the other needle into them.
5. Repeat as needed to add as many stitches as required.
The first slip knot loop counts as a stitch, so make sure to count all of them again once you think you’re done. You may find you have one extra!
A word of warning: although I recommend this casting-on method for beginners, it has some limitations. Specifically: it works for yarn that’s slightly fuzzy and can grab onto itself, like wool or basic acrylic. If the yarn you have chosen is very slippery, like mercerized cotton, you will find it very difficult to knit the first row without the stitches coming apart. Best to stick to beginner-friendly yarn until you get some more experience with different casting-on methods!
Thanks for reading and I hope this is helpful!