Locate the various source code repositories (generally found on GitHub either in the Alfresco or Activiti organisations)
Prepare your development environment based on any documentation that these projects may include
Use Git to checkout the source code
Figure out and switch to the correct branch / tag that you want to build (each project has its own versioning, there is no global "6.0" tag anywhere)
Use Maven (or another build tool if required by the specific project) to execute the build
Deploy, test and pray that you've done everything correctly
While being open source means you "can" technically build Alfresco from source, little effort has been put into making it an easy exercise, especially given how the various sub-projects have now been modularised. There is no step-by-step documentation anywhere for building the entire platform / portfolio, so you will have to figure out a couple of details yourself.