I was taught very briefly at my college.
You prep the nail up to removing the shine. Then you cut the silk to the shape of the nail, and glue it onto the nail. Once it's dry, you file off the excess silk sticking out of the free edge, and then you lightly buff, and add one more coat of glue. After that, it's your choice to whether add some filler powder to that.
It's good for repair of cracked nails, but I'm not really a fan of it unless the client is going to have a full coverage colour on top. If you leave it clear you can sort of see the silk.
It's also apparently good for people who have developed an allergic reaction from gels/acrylics due to over exposure and cannot get them done. My college instructor was like that, so she only had silks