Registration and other formalities differ between programs. Therefore, you will typically need to consult your bachelor or master coordinator to bring about the registration. You will normally also need to find a supervisor and and an examiner. For hacking projects at the NSE department, you can talk to Prof. Pontus Johnson about supervision, and Mathias Ekstedt or Robert Lagerström about examiners (for some programs, examiners may be appointed without your involvement).
Different programs will demand different information, specifications or project plans as a basis for approval of the thesis project.
There is ongoing work at the EECS school to harmonize the formal process regarding master thesis projects. More information is available here.