Depends on the gear piece. You can't do "all Crit" or "all DET" due to stat cap restrictions. What you're capable of melding is dependant on what stats are already present on the gear. Crit has the highest stat weight for secondary stats on NIN, but you won't always be able to meld more Crit.

The reason there's no guides is likely because there really isn't a lot of "choice" in melding. As long as your Accuracy is ok, you meld your best stat until it is capped on that piece of gear, and then move onto the next stat.