Step 1: Create an organization within the UN with a mandate to prove a foregone conclusion, as stated at the UN website: The IPCC was founded in 1988 by the World Meteorological Organization and the UN Environment Programme with a mandate to produce accurate, balanced assessments about human-induced climate change
Step 2: Allow governments to appoint scientists who agree with the IPCC mandate to prove human-induced climate change. Skeptical scientists about human-induced climate change need not apply.
Step 3: Allow the scientists in "working groups" to produce their reports on the science behind the mandate (but go ahead and write the Summary Report for Policymakers beforehand). IPCC in secretive process selects the most sympathetic scientists as "lead authors".
Step 4: The scientists conclude that they really can't make any valid assessment of the relative confidence of the IPCC climate models and feedback assumptions because they haven't done a comprehensive set of observational tests (i.e. haven't compared the models to actual data) to find out if the models are any good. In the final paragraph of this critical section of the AR4 WG1 Chapter 8 page 52 the scientists state that a number of diagnostic tests [of the models] have been proposed, but few of them have been applied to the models currently in use. The inconvenient truth is that this has been done by John Christy, Richard Lindzen, et al, and the models have consistently failed miserably, but those scientists weren't part of the invited "consensus". Next it says it isn't even clear which diagnostic tests are critical to assess confidence in the models. The section concludes by saying that the things necessary to assess confidence in feedbacks simulated by different models have yet to be developed. In other words, the scientists can't make any assessment whatsoever of confidence of the models at the heart of the IPCC "consensus" on anthropogenic global warming.
Step 5: Ignore what the scientists said and in the only part most people read, the Summary for Policymakers, written by bureaucrats in advance of the scientific working group 1 report, state that our understanding of human-induced climate change has improved to the point that we give very high confidence (defined as at least 90%) that humans are responsible because the computer models mandated to say so do indeed say so. And to make it look extra impressive, throw in an exact number from one of the GIGO computer models for human forcing with specified confidence limits to make it look like we have actually done those diagnostic tests the scientists in Working Group 1 said have yet to be developed.