Even without the foreskin, you'll want to ensure that you use soap that won't irritate your penis. Opt for a soap or body wash that is mild and fragrance-free. Ask your doctor or dermatologist about choosing soap that won't irritate your skin. Once again, it's important to choose a water temperature that won't scald or irritate your skin. Aim for warm (but not hot) water, and wash with soap all over your body like you normally would. Lather your mild, unscented soap well between your hands. Apply it to the testicles, the base and shaft of the penis, and under the head of the penis. Even without foreskin, it's important to properly wash under the head of the penis as sweat, bacteria, and debris can still accumulate there.  In the absence of the foreskin, all you have to do is lather the penis and thoroughly rinse away the soap under the shower or in the bathtub. Be sure to thoroughly dry the penis after showering or bathing. Without foreskin, it's technically safe to apply talc or body powder, but you should still exercise caution to avoid getting talc in the urethra or causing skin irritations.

TL;DR in Vietnamese:
Sử dụng xà phòng dịu nhẹ. Tắm rửa. Rửa dương vật.