SB 3.28.21

sañcintayed bhagavataś caraṇāravindaṁ
 vajrāṅkuśa-dhvaja-saroruha-lāñchanāḍhyam
uttuṅga-rakta-vilasan-nakha-cakravāla-
 jyotsnābhir āhata-mahad-dhṛdayāndhakāram
Synonyms: 
sañcintayet — he should concentrate; bhagavataḥ — of the Lord; caraṇa-aravindam — on the lotus feet; vajra — thunderbolt; aṅkuśa — goad (rod for driving elephants); dhvaja — banner; saroruha — lotus; lāñchana — marks; āḍhyam — adorned with; uttuṅga — prominent; rakta — red; vilasat — brilliant; nakha — nails; cakravāla — the circle of the moon; jyotsnābhiḥ — with splendor; āhata — dispelled; mahat — thick; hṛdaya — of the heart; andhakāram — darkness.
Translation: 
The devotee should first concentrate his mind on the Lord’s lotus feet, which are adorned with the marks of a thunderbolt, a goad, a banner and a lotus. The splendor of their beautiful ruby nails resembles the orb of the moon and dispels the thick gloom of one’s heart.
Purport: 

The Māyāvādī says that because one is unable to fix his mind on the impersonal existence of the Absolute Truth, one can imagine any form he likes and fix his mind on that imaginary form; but such a process is not recommended here. Imagination is always imagination and results only in further imagination.

A concrete description of the eternal form of the Lord is given here. The Lord’s sole is depicted with distinctive lines resembling a thunderbolt, a flag, a lotus flower and a goad. The luster of His toenails, which are brilliantly prominent, resembles the light of the moon. If a yogī looks upon the marks of the Lord’s sole and on the blazing brilliance of His nails, then he can be freed from the darkness of ignorance in material existence. This liberation is not achieved by mental speculation, but by seeing the light emanating from the lustrous toenails of the Lord. In other words, one has to fix his mind first on the lotus feet of the Lord if he wants to be freed from the darkness of ignorance in material existence.