In one of my previous posts, I've mentioned that someone who claims "promoting is an art" should be fired. Promoting should be well described and followed consistently.
What I find descriptive as well as very easy to follow is a career path prepared as a maturity model. You can simply, yet extensively describe what is expected from a given position. Is it the ability to lead the development of large features? Or somehow improve the processes and the environment in your company? Even the participation in hiring or customer calls may be required from a certain level of seniority.
In my practice, a candidate for the promotion should fulfill at least a part of the requirements from the higher position, so it's not that much of a shock for him. Also a description of higher positions, like seniors, principals and architects tend to be too wide, so you need to choose a subset. It may be even unachievable. It's extremely important to make these rules transparent and generally known by the team.
Although there is a general career path, I always prepare them individually with a description of the next steps or necessary goals. I as well as the candidate for the promotion evaluate progress usually during performance reviews or whenever it's necessary.
It's great when you can cooperate with your team on creating this model. Let them also share their expectation from more senior colleagues. You'll most likely figure out their expectation from technical seniority is way more demanding than would be yours.
A difficult discussion comes up when the promotion is fair and all the agreed requirements are fulfilled but it's somehow not possible: there can be a freeze on any promotion in your company, expected wage increase may be unavailable due to financial situation or there's simply not any space for e.g. an extra manager. There are two things you can do. Never promise anything you don't have completely in your hands. And be always fair and openly share the current situation. Outline the next steps and possible scenarios and show your personal interest. This will at least mitigate the unfortunate situation.