Get the sources

You will need gnu binutils and gcc. They are available on ftp.gnu.org or any gnu mirror. Let's unpack sources:

$ mkdir src
$ cd src
$ tar xjf ..../binutils-2.xx.tar.bz2
$ tar xjf ..../gcc-x.x.x.tar.bz2
$

Compilation

Now lets compile. I assume we'll need to compile more toolchains later, so let's share the sources and compile in a separate directory:

$ cd ..
$ mkdir build build/mipsel build/mipsel/binutils build/mipsel/gcc

First, binutils

$ cd build/mipsel/binutils
$ ../../../src/binutils-2.x/configure  --prefix=/where/to/put/it --target=mipsel-unknown-elf \
  --program-prefix=mipsel-unknown-elf-
$ make
$ make install
$

Export the bin directory of your installation path, if not already in it. You should make this permanent by setting the PATH variable in a shell's startup script.

$ export PATH=$PATH:/where/to/put/it/bin
$

Now gcc

$ cd ../gcc
$ ../../../src/gcc-x.x.x/configure  --program-prefix=mipsel-unknown-elf- --prefix=/where/to/put/it \
  --enable-languages=c --target=mipsel-unknown-elf --host=i686-pc-linux-gnu
$ make
$ make install
$

We're done !