Vaishnava Tilak–Marking your body as the temple of God
In the Vaishnava tradition devotees wear the urdhva pundra made of gopi chandan commonly known as tilak.
"Tilaka means victory personified." (SPL to Tilaka devi dasi,)
By wearing tilak not only does one identify ones body as the temple of the Lord but also one is blessed by the auspicious protection of the Lord. Not only is the wearer immensely benefited but even those who see the tilak marks are benefited.
“In Kali-yuga one can hardly acquire gold or jeweled ornaments, but the twelve tilaka marks on the body are sufficient as auspicious decorations to purify the body. “ SB 4.12.28 Purport
Srila Prabhupada emphasized that tilak was part of the essential dress of a devotee.
“I have no objection if members of the Society dress like nice American gentlemen; but in all circumstances a devotee cannot avoid tilaka, flag on head (shikha) and (tulasi) beads on the neck. These are the essential features of a Vaisnava." (SPL to Brahmananda, 14th October, 1967)
Tilak is applied with the Lord’s holy names thus giving protection to the whole body.
“While decorating the body with tilaka, we give protection to the body by chanting twelve names of Vishnu. Although Govinda, or Lord Vishnu, is one, He has different names and forms with which to act differently.” SB 10.6.27-29 Purport ACBSP
"Persons who are decorated with tilaka or gopī-candana [a kind of clay resembling fuller's earth which is produced in certain quarters of Vrindāvana], and who mark their bodies all over with the holy names of the Lord, and on whose necks and breasts there are tulasī beads, are never approached by the Yamadūtas." The Yamadūtas are the constables of King Yama (the lord of death), - Nectar Of Devotion Ch. 9
Srila Prabhupada has given the example of a policeman. He can be identified by his dress. Downing the dress, the policeman is also reminded that his behavior should honor his position.
Simply by seeing someone wearing tilak, people are induce to utter the words, “There is a Hare Krishna” and thus they immensely benefit thus we should not see it as something external.
“Just like sometimes on the street some outsider, seeing you, they chant Hare Krsna only by the symbolic, sānketya. Because they see: "They have got tilaka, kunti." Therefore these things are required. Don't become immediately paramahamsa—no tilaka, no kunti and no bead bag. This is not good. Sānketya. So that others may understand, "Here is a Vaisnava. Here is a Krsna devotee..." And if he is simple, he'll chant, "Hare Krsna." This chance should be given. Therefore it is necessary, how people can utter. That chanting may save him from the greatest danger. Therefore it is said, sānketyam pārihāsyam. If somebody jokes... Sometimes they do that. "Hare Krsna" means he is not seriously chanting, but he is trying to joke the other party who is engaged in chanting. And that is also good, pārihāsya.” - Śrīmad-Bhāgavatam 6.2.13 -- Vṛndāvana, September 15, 1975
Here is a quick video tutorial of how to put on tilak.