Start With First Principles

Avoid starting from a prior conclusion or forming an opinion first and then working backwards to justify the conclusion or opinion. Define your axioms and principles first - these are your first principles - and then let them guide you as you build your arguments towards a conclusion or opinion. Focus on addressing things at the root cause level.

People don't work out what they think and then join the corresponding tribe. They join a tribe and infer from it what they think. - Janan Ganesh

A sign that you may not be doing this is if your beliefs tend to cluster based on a group identity. For example, if knowing one stance on a topic allows for prediction of other stances, then you may not be doing your own thinking from first principles.

The 2nd most robust intelligence test: check how much one's belief cluster, particularly on uncorrelated matters. The most stupid hold beliefs that are negatively correlated. - Nassim Taleb