I am reading the knot book from Colin Adams, and after he discussed how one may construct a (Seifert) surface whose boundary is the knot, he went on and stated that the figure 8 knot has genus 1 (that is, it has a seifert surface which can be "capped" off with a disk to obtain the torus, and he stated it without any explanation as if that's an obvious fact - which is not to me.
In general how might one expect to "capp" off the disk of a seifert surface? Especially when the boundary is any non trivial knot, it seems really hard for me to visualize how one would obtain one of the orientable compact surfaces by capping off the boundary, heck It's hard for me to even visualize how we might get a surface in 3d without any self-intersections.
Edit: Colin Adams not Adam Colins