Short answer is "you can't."
When in aperture priority, you control aperture only, and the camera controls the shutter speed.
When in shutter priority, you control the shutter speed only, and the camera controls the aperture.
That's why these are sometimes "semi-automatic" modes - you control part(s) of the exposure triangle, and the camera controls the rest.
If you want to be in charge of both aperture and shutter, you need to be in Manual mode. Sounds like you're ready to give Manual mode a try.
