Well, I'll take a shot at it:
1) download sigil 0.7.1 source code zip file from
here (left column under "downloads")
2) download Qt5 install package for your architecture from
here and install (as root) to /opt/Qt5.0.1 or as normal user, to $HOME/Qt5.0.1 for example.
3) If you don't already have them, install development tools (build-essentials, cmake, etc.)
4) follow instructions in the "runningfromsource" sigil wiki page
here (scroll down to Linux).
Good luck!
Albert