This may be the most important piece of advice, because it affects all others.
Being first-handed means observing reality with your own eyes/brain, forming your own judgments, and acting on them with integrity. It is a primary orientation toward reality, not other people. Put negatively, it means not accepting ideas passively or on the authority of others. This is difficult to do consistently.
For example, you hear someone claim that “97% of all scientists accept that CO2 is causing catastrophic global warming.”Even if true (it’s not), you do not accept its truth based on the authority of the scientists. (And you do not accept my claim passively, either.) You go look at the observation-based evidence, including the credibility of the scientists who claim knowledge.
(The “fist-handed” part is half in jest, but those who are second-handed tend to resort to the fist or allow it passively. For example, those whose authority is God or The People [the Volk] or The Environment, like ISIS, Hitler, and Al Gore.)