A good way to learn is to try to answer user questions. Instead of referencing other posts, I tried today to answer that frequently asked question myself on developpez.com (french forum)
SQL> create table test ( my_long long);
SQL> insert into test values ('hello toto !');
1 row created.
SQL> exec for r in ( select my_long from test )
loop if (r.my_long like '%toto%') then
dbms_output.put_line(r.my_long); end if;
hello toto !