Estimating the entire project upfront can be a daunting task, but there's a technique you can use when faced with any big problem - break it down into many small problems!
Your first step would be to look at a list of all features and pieces of content. Think of any design tasks that could go on that list. For example, if you're working on a first-person shooter, you'd not treat level design, weapon design, and enemy design as just three huge tasks; instead, weapon design could be broken down and estimated as a set of smaller, more manageable tasks:
- High-level design of all weapons (quantity, when are they unlocked): 2 days
- Design and implementation tasks for each weapon: 3-4 days (per weapon)
- Ideation and prototyping ...
