I'm not sure if this is a spoiler, but you can combine glyphs if they have at least one of the same words when you right-click them. This means that if you right-click your fire glyph in your inventory window, it might say something like "This glyph can be interpreted as fire, flame, heat, etc". Now, if you have another glyph and you right-click it, let's say: "This glyph can be interpreted as blast, heat, wind", then you can combine those two glyphs because they have "heat" in common. This is just a theory, and it may not always work since other glyphs can also have that "Heat" attribute in their description, meaning you can only combine those glyphs if all 3-4 of them are being combined together.
Hope that helps.