Often there is a conflict between Dharma Sastras and the laws of the lands. So, how can these verses to be treated ?
As noted in this answer, a Smriti can be divided to have three distinct sections (Kanda-s):
- Achara Kanda 2. Vyavahara Kanda 3. Prayaschitta Kanda
Achara Kanda contains the rules and prescriptions regarding rites and rituals, rules of initiation, rules for purification etc.
These rules (at least most of them) can surely be practically followed even in today's time.
Vyavahara Kanda contains the civil laws and allied things. These rules obviously can not be followed. Because the laws of the Government should be followed instead.
The Smritis prescribe punishments based on castes of the guilty persons. But the laws of the country will not of course bring castes into consideration while judging.
Manu Smriti 8.267. A Kshatriya, having defamed a Brahmana, shall be fined one hundred (panas); a Vaisya one hundred and fifty or two hundred; a Sudra shall suffer corporal punishment.
8.337. In (a case of) theft the guilt of a Sudra shall be eightfold, that of a Vaisya sixteenfold, that of a Kshatriya two-and-thirtyfold,
8.366. A (man of) low (caste) who makes love to a maiden (of) the highest (caste) shall suffer corporal punishment; he who addresses a maiden (on) equal (caste) shall pay the nuptial fee, if her father desires it.
Also, death penalty for defiling a maiden, and various other similar punishments, will not be permitted as per the Government laws.
The Prayaschitta Kanda contains various methods for atoning for committed sins. These rules also can be followed provided the person is capable and willing. The question of a person's willingness and capability is important as some these atonement measures are very cumbersome to perform. Except when charity is the atonement.