Expanding on zcp's suggestions:
A) Understand the application and the nature and magnitude of the loads.
B) Determine an appropriate safety factor.
C) Select a weld joint capable of sustaining A x B.
D) Design a weld procedure capable of consitently producing the weld selected in step C.
E) Qualify the weld procedure by load test of the assembly, and some combination of weld sectioning, radiographic, ultrasonic, DP or MP testing.
F) Qualify as many welders as you need for that procedure.