Practical questions for having /tmp and /var on same partition

Hi.

I just installed MX Linux onto a computer, and tough the installer does provide an option to have separate / /root /home and swap partitions, it does not natively offer the user to specify partitions to put /var and /temp.

Therefore, I have set aside 25BG free space for this purpose. I also want to put /var and /tmp on same one partition.

On this page - unix.stackexchange.com/../why-put-things-other-than-home-to-a-separate-partition it says:
Quote:

var and /tmp can be filled up by user programs or daemons. Therefore it can be safe to have these in separate partitions that would prevent /, the root partition, to be 100% full, and would hit your system badly. To avoid having two distinct partitions for these, it is not uncommon to see /tmp being a symlink to /var/tmp.

The first practical question - what steps are necessary to do this? According to last post in this askubuntu forum thread:
Quote:

if you provide an existing directory instead of the link name, ln will create a link inside that directory

I've made symlinks before, but this confuses me.

Thanks in advance