To locate it precisely:
1. Consult your owner's manual: This is the best resource. The manual should have diagrams showing the location of major components.
2. Look near the intake manifold: It's typically on top or to the side of the intake manifold, relatively easily accessible once you've removed some air intake components. It's often a small, rectangular or cylindrical valve with a vacuum line connected to it.
3. Use online resources: Search for "1999 Chevy K1500 [engine size] EGR valve location" on Google Images. You may find pictures of the engine bay showing the valve's location.
Caution: Working on your engine requires mechanical aptitude. If you're unsure of what you're doing, it's best to take your truck to a qualified mechanic. Improperly handling engine components can lead to damage or injury.