How can I learn Imperial system I using the metric system my whole life?

Why do you need it?

To the point though - practice. It is totally illogical so you just must memorize most of it. You might learn some of the history to give you some context, but it is a small help and you will have to brut force it.

My context for "learning" Imperial is solely based on measurements of length. I've "learned" it b/c i listen to a lot of US-Centric carpentry podcasts. I've just memorized 1 inch ~ 2.54 cm ; 3/8 inch ~ 1 cm. Most other fraction can be derived from those in the head. At least enough for following a carpentry podcast.

When i need more precision, i use an online converter or a ruler with an imperial and a metric scale. FWIW i only think in metric.