I am attempting to install a bidet attachment I ordered online. The first step is to shut off the supply valve and empty the toilet, as it attaches on between the supply valve and tank. I am unable to fully shut off the water supply. The supply valve opens fully and it will stifle the supply water somewhat but not completely off it. I am afraid to force it or use a tool with my level of plumbing knowledge. Could any helpful people give me advice on a course of action? I see a screw on top of the valve— I’m deathly afraid of losing some piece of it and flooding the bathroom. As much as I can hand turn it, it just isn’t enough to cut off the supply to do the work needed. I could shut off the entire house water supply if all else fails but I’d like to be able to turn off the water supply valve locally.