Hey SolidWorks Community!

I'm a first time poster so I'm excited to learn from all of you gurus out there...

I can't seem to figure out how to cut into a boss that I have extruded from a circle sketch... it now looks like a cylinder...

I tried to sketch onto a plane that is tangent to the cylinder and use an extruded cut as a "cookie cutter" to go into said cylinder, but the 3D sketch isn't going in the correct direction... I need the shape of the sketch to go straight down a certain depth from the plane...

I have added a picture if I did a horrible job explaining...

HELP ME, SOILDWORKS COMMUNITY. YOU'RE MY ONLY HOPE.

-Kazuki