Okay not so much as you can edit it as run it
Some people do not have access to root so their server provider is only able to do this for them but if you have root access it should be something like this
go to root level directory something like this
/var/spool/cron and in their there should be a file named usually your site name
Edit this file and put a line something like this format
* * * * * /PATH/TO/subscriptions.php
The asterik's are this in a row.
minute(s) hour(s) day(s) month(s) weekday(s) then the command to run as seen.
Now this is the crontab file.
Telnet into this directory and usually
cd /path/to/var/spool/cron
Now this is a bit different on servers I have seen but on mine I run this
crontab -u filename filename
then next
crontab -u filename -l
Make sure to leave the spaces where they are. Thats it it should be installed now.
