1. Get all info possible.
2. Usually, the process that you are designing with comes with documentation containing stuff like vt, lambda etc, and also how they vary with process and temperature.
3. Since in MOS design, you can design almost everything with ratios of W/L, we fix the ratios, etc. ( have to to come up with the schematic first)
4. Now, fix L. L is kept the same for convenience, though its easy to find some places where the length is varied over a wide range.
5. Find L from the lambda parameter and the currents that flow through the transistor.
6. Iterate.
Hand calculation results aren't that far from normal results as far as i've seen.