I don't see what your problem is? - of course outputs go to inputs - if you want an active mixer then use an opamp, but from headphone outputs you might have enough signal to use a passive one.
The essential principles of mixing audio signals are straightforward, but strangely are not explained very well at all on the Net. This article should fill the gaps.
sound-au.com
An active 'virtual earth' mixer is always better, but you could try the passive one with just pots and resistors