The first time I felt like I was really beginning to understand tapestry was while practicing hatching. This technique is when more than one weft thread travels into the horizontal space of another, creating layers of interchanging weft threads.
Hatching is great for blending colors and is a good technique for beginners to help them to understand how warp and weft relate in tapestry weaving.
In the picture here, the top part of the weaving (the blue green and purple) is hatching.
To start hatching, first weave two wefts in the same shed (so in the same line) facing opposite directions. This means the loose ends of the wefts are going different ways.
If you want to start with three or more wefts on the same line, you can do that too, making sure one weft is going one way, the next the other, the next the same way as the first, etc. I will show you how to do this with just two weft threads, but the concept is the same for more than two.
After you have started your two weft threads in opposite directions, take your left weft thread and bring it over on top of the right one as far as you'd like. If you are using the shedding device, make sure you change your shed before doing this.
The next step isn't as simple as weaving the right weft back over. If you try to do that now, you will both cover the end of the other weft and you'll be in the wrong shed once you cross over. See below for a visual of why this doesn't work.
Instead, you need to weave three more passes. One pass will bring your right weft to the edge of the left weft. The next will bring it to the edge of the piece.
On the third pass, you can bring your right weft over the left one!
That's it! Now, do the same thing with the left weft thread. You can bring the wefts as far over on top of each other as you'd like to create different effects. Try it out!