The procedure is quite the same as for gcc 4.8 as you can see in my older post Installing gcc, g++ and gfortran 4.8 from source
Read the manual.
Download, unpack, switch dir, download, unpack, link.
$ wget ftp://ftp.gwdg.de/pub/misc/gcc/snapshots/LATEST-8/gcc-8-20190118.tar.xz
$ xz -d gcc-8-20190118.tar.xz
$ tar xf gcc-8-20190118.tar
$ gcc-8-20190118/
$ wget ftp://ftp.gmplib.org/pub/gmp-6.1.2/gmp-6.1.2.tar.bz2
$ tar xjf gmp-6.1.2.tar.bz2
$ ln -s gmp-6.1.2 gmp
$ wget https://www.mpfr.org/mpfr-current/mpfr-4.0.1.tar.bz2
$ tar xjf mpfr-4.0.1.tar.bz2
$ ln -s mpfr-4.0.1 mpfr
$ wget https://ftp.gnu.org/gnu/mpc/mpc-1.1.0.tar.gz
$ tar xzf mpc-1.1.0.tar.gz
$ ln -s mpc-1.1.0 mpc
Make a build directory, start configure, start make, wait
$ cd ..
$ mkdir build-gcc-8-20190118
$ cd build-gcc-8-20190118/
$ ../gcc-8-20190118/configure --prefix=/opt/gcc-8 --enable-threads --enable-languages=c,c++,fortran
$ make -j 32
# make install