You should not edit the crontabs files in the /var area. These are auto-generated, and will probably get over-written.
If you want to edit the crontab for any user, log in as that user (or su to it), and then use the command:
You can set the EDITOR environment variable if you want to use an alternative editor, for example:
EDITOR=/usr/bin/kwrite crontab -e
These user crontabs do not have the user name in them. The format would look something like this (example to execute at mid-day):
0 12 * * * command arg arg ...
To change the working directory before executing a command, do it like this:
10 12 * * * cd /new/working/directory ; command arg arg ...
The ability to use multiple commands like this may not work in some older, more conservative implementations of cron. I can confirm that it does work with anacron, YMMV with other cron implementations.
If you want to use the system crontab's (as opposed to the user cron tabs), you can add a script to /etc/cron.daily/
You need no crontab like entry at all - all scripts in /etc/cron.daily are executed once a day.