How to define a software development life cycle?

Defining a software development lifecycle (SDLC) is a complex task, but it can be done by following several simple steps:
1. clearly define the scope and purpose of the SDLC
The scope and purpose should be documented and presented to the management of the organization and to the people that will be affected by the SDLC. Getting a wide acceptance for the initiative is probably the most important factor for success.
2. establish a strategy for SDLC definition using one of the following commonly used ones:
- SDLC improvement
- SDLC reengineering
- new SDLC adoption
The strategy should be chosen based on the desired magnitude of the change (SDLC improvement has the lowest change magnitude, new SDLC adoption has the highest magnitude), on the people available to drive the initiative, on the time constraints, on the acceptable risks for the SDLC definition project, etc.
3. define and document the SDLC
Whenever possible use graphical notations to document the processes that make up the SDLC.
4. validate the SDLC for several pilot projects
It’s a good time to see what is working, what have to be fixed or enhanced and to get the success storied to be later used for marketing the new SDLC.
5. release the SDLC
All the people that will be affected by the new SDLC will have to attend extensive trainings to get both a general understanding and a job specific understanding of the SDLC.
6. continuously improve the process
SDLC definition is a never-ending task so it should be continuously improved an d adapted.

Throughout the SDLC definition project, communication plays a vital role for the overall success of the initiative. All the people that will be affected by the new SDLC should be kept in-loop with details on the progress and on the new SDLC. They should be constantly asked for feedback.
Whenever possible, reuse! A lot of things have been discovered and documented about software development processes so instead of re-investing the wheel check if it’s not already available. ISO standards like ISO12207 and ISO15288, standard process models like Waterfall Model, V-Model, Agile Models, etc process improvement approaches like CMMI, etc, should be consulted while defining the process.

Please check some SDLC secrets in SDLC 100 Success Secrets - Software Development Life Cycle (SDLC) 100 Most asked Questions, SDLC Methodologies, Tools, Process and Business Models.