Sometimes with knitting patterns, it's best to just start following the instructions and see what happens. So with the instructions you wrote, knit 12 stitches, then knit 4 stitches together, then knit 12 stitches, then knit 4 stitches together, (keep going like this).
If you already understand how to knit 4 stitches together, then try just slowly and carefully following the pattern. It will take some time before reading a pattern will make sense visually in your mind's eye, so don't expect that yet!
I often knit a small swatch in the pattern, and then I can see what's going on before starting on the real-deal. A typical swatch is 20 stitches wide and 40 rows tall, making a rough square. But this may vary depending on the individual pattern.