You contact a particular creature with which you are familiar of 25 words or less, including the suggestion. The creature can also give a short reply immediately. The subject recognizes you if it knows you. It can answer in like manner immediately. 

The message can also contain a suggestion. The suggestion must be worded in such a manner as to make the activity sound reasonable. Asking the creature to do some obviously harmful act automatically negates the effect of the spell.

The suggested course of activity can continue for the entire duration. If the suggested activity can be completed in a shorter time, the spell ends when the subject finishes what it was asked to do. You can instead specify conditions that will trigger a special activity during the duration. If the condition is not met before the spell duration expires, the activity is not performed.

The demand, if received, is understood even if the subject's Intelligence score is as low as 1. If the message is impossible or meaningless according to the circumstances that exist for the subject at the time the demand is issued, the message is understood but the suggestion is ineffective.

If the creature in question is not on the same plane of existence as you are, there is a 5% chance that the demand does not arrive. (Local conditions on other planes may worsen this chance considerably.)

A very reasonable suggestion causes the save to be made with a penalty (such as –1 or –2).