To make a plan, first, we have to estimate the Product Backlog Item (PBI) because when you know how “big” a PBI is, you can make your high-level plan.
The most used measurement unit is the Story point (or even ideals days), which is just a number. You need not stick this number to a precise number of days because you want a high-level plan without getting stuck on it. The planning goal is “to make a prevision” instead of “to commit to it.”
If you have the story point of the item, you need to know how many stories points your team can make each sprint; that is the velocity of the group. You can start with an ideal estimation and make an average for every sprint by calculating how many stories the team made every sprint (the sum of the item that met the definition of done x the story of each of one).
If you have your ordered backlog, know each PBI how many story points require, then you can make a high-level plan with the item you can release for each sprint.
Easy Guide for Project Management