Hi,
To restrict the users to can not see other users instances select the below options in Access level or particular folder access.
View document instances (Denied)
View document instances that the user owns (Granted)
With these options all users can only see there scheduled instances.
Before updating the security make sure these changes are aligned with your existing security.
There are multiple ways to achieve this.
1) Create two user groups and assign security according to that.
2) If this is going to applicable for all users in the system then you can select these options in the access level itself.
3) If this is required for one particular folder then you can select these options in advance tab in the assign security.
Regards,
Amit