Anatomy of an encounter table

Bandits and badgers and owlbears, oh my.

There are a few considerations when designing random encounter tables, including theme, probability and challenge level.  Random encounters are a useful way to provide theme and immersion. You can include creatures unique to the area, from relatively harmless wildlife to infamous monsters.
Probability ranges should match the relative rarity of potential encounters. The range of challenge level may vary more in a table designed for a large area than one for a small locale, but should usually be reasonably appropriate for the current party level.

You might allow survival or relevant knowledge checks to modify the random encounter result. Depending on whether characters are actively searching for dangerous quarry or attempting to avoid challenging hostilities, this can be a plus or minus to the number rolled on a table suitably arranged for this purpose. The DC for this can be 10+ the average CR of the table, with each +5 over the target number allowing an additional adjustment (from +/-1 for a D20 table, to +/-5 on a D100).

Remember that the more dice you use in the roll, the more you drive probability toward the center. For example, the chances of a roll on the classic 3D6 bell curve can vary from about 1% for the lowest and highest, to about about 12% at mid-range. So if your forest is infamous for its dire badger dens, you should probably set that critter’s sett in the center.

Temperate Forest Encounter Table (Average CR: 3)