How to use Oracle global temporary table?

I am attempting to use an Oracle global temporary table without physically creating a table in the database. The following code is not working. Can someone please explain the proper way to use global temporary tables?

declare
  global temporary table my_temp_table(column1 number) on commit preserve rows;    
begin
  insert into my_temp_table (column1) values (1);
  select * from my_temp_table;   
end;