You can't, natively. Jira doesn't do field level security.
The feature below was created to request this, and it was closed as 'Won't Fix'.
I would recommend to check the issue above for more details about the decision for Atlassian not implementing the feature, as well to check the workarounds provided in there.
You may also check the Field Security Plugin from Quisapps, as mentioned by Nic.
This other question also provides some additional alternatives to achieve what you want:
In the past, Portfolio for Jira required a high degree of detail–foresight that was unrealistic for many businesses to have–in order to produce a reliable long-term roadmap. We're tur...
Connect with like-minded Atlassian users at free events near you!Find a group
Connect with like-minded Atlassian users at free events near you!
Unfortunately there are no AUG chapters near you at the moment.Start an AUG
You're one step closer to meeting fellow Atlassian users at your local meet up. Learn more about AUGs