Metrical psalms are arragned to be sung in personal, family, or corporate worship. If you've ever sung a hymn, you're prepared to sing a psalm.
"And for the English Tunes that we use in singing of Psalms, take this for a ground; Since God hath commanded us to sing Psalms and Hymns and spiritual Songs, and amongst others, those of David: and yet withal hath hid from us the Hebrew Tunes, and the musical Accents wherewith the Psalms of David were wont to be sung. It must needs be that the Lord alloweth us to sing them in any such grave, and solemn, and plain Tunes, as do fitly suit the gravity of the matter, the solemnity of God’s worship, and the capacity of a plain People."
- John Cotton, Singing of Psalms a Gospel-Ordinance
Meter refers to the arrangement of syllables and stanzas in a given psalm. "86.86" means that the number of syllables per line alternates eight, six, eight, and six.
A - maz - ing grace how sweet the sound (eight)
That saved a wretch like me (six)
This psalter only uses a few meters:
Common Meter: 86.86
Long Meter: 88.88
Short Meter: 66.86
148th Meter: 66.66.44.44
Once you know your meter, you're ready to pick a tune. Here are some favorites.†