When I try to build the kernel I get an error telling me to run "make mrproper" what should I do?
This implies there are left over files in your tree which are not meant to be in a clean tree. A clean source tree is required for an out of tree (make O=<directory> style) build, which is what the Ubuntu packaging uses. The most common cause is the creation of the include/config directory. You can try removing this:
If that does not work it is normally safest to clean all extraneous file out of your tree as below. Note that this removes each and every file which is not committed to git, it will remove any patches etc you have lying around within the build directory:
git clean -x -f -d