They are there. DMG pg 76 and before. The process is a skill check; from the SRD:
Craft DCs For Mechanical Traps
Once you know the Challenge Rating of a trap determine the Craft (trapmaking) DC by referring to the table and the modifiers given below.
Trap CR Base Craft(Trapmaking) DC
1-3 20
4-6 25
7-10 30
Additional Components Modifier to Craft(Trapmaking) DC
Proximity trigger +5
Automatic reset +5
Making the Checks
To determine how much progress a character makes on building a trap each week, that character makes a Craft (trapmaking) check. See the Craft skill description for details on Craft checks and the circumstances that can affect them.
Etc.
