How to change Custom.pll File and How to compile It

Compile Command:- frmcmp_batch module=CUSTOM.pll userid=apps/A9958Alj17 module_type=LIBRARY output_file=CUSTOM.plx Compile_All=special Batch=NO


=============This is custom Code to add in  Custom.pll file =====================   
    --Copy invoice amount to release amount net amount field
   
    if (form_name = 'APXINWKB') then
        if (block_name = 'INV_SUM_FOLDER') then
            if  (event_name='WHEN-NEW-ITEM-INSTANCE') then
             
             
               if name_in('INV_SUM_FOLDER.INVOICE_TYPE')='Retainage Release' then
                        lv_invoice_amt := name_in('INV_SUM_FOLDER.INVOICE_AMOUNT');               
                   copy(lv_invoice_amt,'INV_SUM_FOLDER.RELEASE_AMOUNT_NET_OF_TAX_DSP');
                        copy(lv_invoice_amt,'INV_SUM_FOLDER.RELEASE_AMOUNT_NET_OF_TAX');
                       
               end if;        
            end if;
        end if;
    end if;