Learn from past estimates
The development team uses retrospective meetings to discuss knowledge gained from past iterations as well as to review the accuracy of their estimations. Agile project management tools (like Asana or Jira) enable the tracking of story points, thus enabling the team to reflect on past estimations and make necessary adjustments, with relative ease.
For example a team could pull the last 7 user stories that the team has developed with a story point value of 8. Further, discuss each of the items, check if they had the same level of effort (when actually developed). If the answer is no, the team can discuss why, the information gained can definitely help the team with future estimations
Like many aspects of the agile process, estimation is a practice. Yourself and your team will improve with time.