How to join yarns without a knot: Russian join step-by-step
Broadly speaking, you have three options for the joins between balls.
The simplest is to tie a loose knot, then, when you finish your garment, you untie the knot and weave in the ends on the wrong side. You can also join as you go with a spit-splice, but this only works with feltable animal fibres such as wool and alpaca. We quite like the Russian join, which we've shown below.
Please note that this method works better with finer yarns but we're using a chunky yarn here to make it easier for you to see what we are doing.
Step 1: Thread Yarn A onto a darning needle
Step 2: Use darning needle to weave Yarn A back on itself.
Step 3: Do not pull tight, leave a loop at the top.
Step 4: Thread Yarn B onto a darning needle.
Step 5: Thread through Yarn A loop, then weave Yarn B back on itself.
Step 6: Pull two ends tight to close loops and trim ends.