An Expert Judgement is very simple when it comes to explaining because it is what it seems it is. What you need to do is to find an expert who did a similar project in the past. Provide him with details about your project or tasks. Mention how big your team is, what technology are you using, and how experienced you are. In the end, just ask for estimates.
Estimate or guesstimate?
You can think that since an expert won’t be involved in the development process, he can just make a guess. He takes no risks.
The crucial point is to find the correct man, with proven experience and correct attitude, who will spend enough time on understanding your project. Even better you can invite multiple experts and facilitate a workshop with them.
Give them enough time to think before you demand an estimate from him. Guesstimate backed up with years of experience is actually an estimate.
When should you use an Expert Judgement?
This works well when estimating the overall cost of a project. For instance, when you have scoped MVP – you can (or you even should) – ask some experts about what they think the overall cost will be.
On the other hand, there is no point in asking an expert for an estimate when it comes to one isolated feature during the development. He will lack many pieces of information, technical debt, or limitations of codebase just to name a few. In such a case his estimate might be off multiple times.