Do Not Learn Metric
You don't need to "learn" metric — just use it. When you were a kid your parents took you to the grocery store and you put a gallon of milk in the shopping cart. And today you know what a gallon is even if it was never explicitly taught to you. Likewise, you know your own height and weight because you measure yourself. Nobody had to teach you how to step onto a bathroom scale.
AFTER you are familiar with using metric, you can learn about prefixes, base units, and orders of magnitude. The metric system is based on a coherent set of principles enabling measuring units to integrate in a logical manner, but you do not need to know that to start your metric journey.
Many people are reluctant to switch to metric due to concern it will cause mental mayhem. It's actually not too difficult. After switching over to Celsius for a week you'll likely have a good intuitive sense of Celsius.
If you get a metric only tape measure and switch over to using metric for your home projects, you'll quickly realize that millimeters are awesome. The mm is the perfect size. A mm is the smallest length you can reliably measure quickly without special tools and extra lighting. Metric literally requires less effort to remember your measurements and helps you quickly cut accurately.
Steps
- Settings — Set your mobile device to metric. Set your refrigerator, home thermostat, and car temperature to Celsius. Set the scales in your kitchen and bathroom to display grams and kilograms.
- Tools — Buy some metric only measuring devices (avoid dual units like the plague because dual units kill your senses). Sadly, you'll have to purchase online as retail stores mostly sell imperial.
- Communication — When talking with other people use metric as if everyone normally uses metric. When you get a puzzled look, just casually provide a frame of reference or gesture a size with your hands.
Do not do unit conversions.
After using metric for a while, you'll realize that the inconveniences you encounter being in an imperial world are pretty much the same as before. Imperial and metric share one important trait — they are both incompatible with imperial.
Every once in a while you'll encounter something like a product page on Amazon where the manufacture or reseller didn't bother to convert to imperial. The product information will be all metric.
IMPORTANT NOTE:
"Nothing to learn" only applies to casual everyday metric usage.
Metric in science, engineering, medicine, publishing, legal, and other professional domains definitely requires knowing the rules of SI (International System of Units).