You have two options: manual or automated.
- Manual: type in a hashtag on Instagram relevant to your niche in the search bar i.e. gym workouts. See what is the most popular and has the most amount of followers.
- Automated: use a tool like Hashtag Expert to generate a list of specific hashtags relevant to your niche. Pick the top 5-10 that best describe the post and your niche.
Et voilà.