The  code  below shows how to programmatically create a custom permission level for a specific SharePoint site :

using (SPSite osite = properties.Feature.Parent as SPSite)
      {
          using (SPWeb oweb = osite.OpenWeb())      
           {
               SPRoleDefinition roleDefinition = new SPRoleDefinition();
                  roleDefinition.Name = "Custom Permission Level";      
                 roleDefinition.Description = "This Permission level has custom set of rights defined within it";  
                   roleDefinition.BasePermissions = SPBasePermissions.AddAndCustomizePages |    SPBasePermissions.ApplyStyleSheets | SPBasePermissions.CreateGroups;                     oweb.RoleDefinitions.Add(roleDefinition);  
               }
             }