What do you mean by customizing?
changing the order or adding the new fields?
Check Page 54 - Purchase Order Subform
What do you mean by customization?
Would you like to change caption of fields,width of the fields,datatype of the fields ???
If you want to change the order of the fields
then right click on any fields and select choose columns Here you can those fields on right hand side
Just use Move Up and move down to order it.
I want to add new fields? 
Go to tables 39 and add the field which you require.
Right click on the Type/No. on Lines subform and select Choose Column…
Check whether the field is available in the list then add directly…
if not, go to page 54 and add from table as existing fields…
Ohh I am sorry I thought adding new fields in tables
how can i go to page 54? sorry if a silly question.
Go to Tools–>Object designer…
Select Page from the list