How to Use

• At the start of an exploration turn (or when a significant action has been initiated but not completed), check for wandering monsters. 
• I like a 10% chance each turn. 
• Roll both the chance of encounter and the particular encounter itself at the same time.
• Keep these rolls secret.  
• Wait until an opportune (i.e., the worst) time to interrupt the action of the turn. 
• Check for surprise.
• Old School Way
•  33% chance on both sides, opposed Stealth vs Passive Perception rolls, whatever.
• If the PCs are surprised but the monsters are not, the PCs stumble into it. 
• If the monsters are surprised but the PCs are not, the PCs get the drop on them. 
• If both are surprised, they stumble into each other, so to speak.
• A Different Way
• Don't worry about surprise as such. 
• When you roll a random encounter, if there are ambush squads out looking for intruders or mindless wandering dangers, then roll an additional die. Shall we call it the gank die?
• If the gank die comes up low, proceed with a gank encounter. It's not supposed to be interesting; it's supposed to be dangerous. Determine surprise the old school way.
• Otherwise, proceed with one of RR's encounters as if the PCs had surprise. 
• Reaction Roll. 
• Do it secretly.
• The way I do them is a bit simplified. 
• There are three categories: hostile, neutral, positive. 
• What these mean is context-dependent. 
• You can determine according to whatever scale you want. Here are a couple that are about mathematically equivalent:
• hostile / neutral / positive
• 2d6: < / 7-9 / 10+
• 1d20: < / 10-17 / 18+
• Initiative, as you like it.