At the moment it is a bit complicated and definitively not straightforward process.
Perhaps the best way is to:
mkdir magnolia
cd magnolia
svn checkout http:svn -N checkout http:cd magnolia
./checkout.sh
cd ..
mkdir enterprise
cd enterprise
svn checkout http:./magnolia-enterprise/checkout.sh
If you are on windows it should be enough to rename checkout.sh to checkout.cmd to make it work there.
By executing the above you should end up with structure like follows:
/magnolia
/magnolia
/<core magnolia + community modules>
/enterprise
/<enterprise magnolia + enterprise modules>
With that in place you should be good to go for maven build.
cd magnolia/magnolia
mvn install
cd ../enterprise
mvn install
After having build all core parts of the CE and Enterprise edition you can go into any of the optional modules directories and build those as well.
This is redundant with Introduction to building Magnolia and Developer information