//Add Permission Sets to Users
List<User> users = [SELECT Id FROM User WHERE username in ('<USERNAME_GOES_HERE>')];
List<PermissionSet> permissionSets = [SELECT Id FROM PermissionSet WHERE Name = '<PERMISSION SET API NAMES GOES HERE>'];
List<PermissionSetAssignment> permissionSetAssignemts = new List<PermissionSetAssignment>();
for(User theUser : users){
  for(PermissionSet ps : permissionSets){
     permissionSetAssignemts.add(
        new PermissionSetAssignment(AssigneeId = theUser.Id,PermissionSetId = ps.Id )
     );
  }
}

System.debug('permissionSetAssignemts-->'+JSON.serializePretty(permissionSetAssignemts));
insert permissionSetAssignemts;