How do I go about to mark the syllables, separate the feet and indicate the rhyme scheme of this part of Keats' "Ode on a Greacian Urn"? When old...

Marking syllables, separating
metrical feet and indicating rhyme scheme are all steps in
scansion, the process by which a poem's
rhythm, meter and rhyme are analyzed. Marking
syllables refers to finding the stressed and unstressed beats of the
rhythm. It's very much like music: some beats of a measure
of music are emphasized while other are softened.

Poetry
has a number of possible rhythms. A common one is iambs.
These have a rhythm of a soft beat followed by an emphasized beat. Poetic
terminology for this is unstressed stressed and may be
typed like this ^ / , with the ^ meaning unstressed and the / meaning stressed. Another
example is dactyls. These have a rhythm of stressed unstressed unstressed / ^ ^. The
words accented and unaccented can be used in
place of stressed and
unstressed.

Look at the first line in
Keats poem: "When old age shall this generation waste." In the first line there is a
problem and a clue to scansion. The problem is that Keats varies the rhythm so that
old age shall this isn't exactly in a smooth rhythm, so save the
first half of the line for last.

The clue is the word
generation. Look it up and you see that its syllabic pronunciation
is gen e RA tion, or stressed
unstressed stressed unstressed, or / ^ / ^ . This means generation
fits either an iamb ( ^ / ) or a trochee ( / ^ ) rhythm. Choose which by scanning the
first line using these marks ' (stressed) ^ (unstressed). Try it both
ways:

When' old^ age' shall^ this' gen' e ^ ra' tion^
waste',
When^ old' age^ shall' this^ gen' e^ ra' tion^
waste',

Scanning the line with an emphasis on
When' creates a problem between this
and gen-. While this can be either
stressed or unstressed, gen- must be stressed. We now know that the
opening line is ^ /, or unstressed stressed, or iambic, not
trochaic.

Now how many repetitions of the pattern, or how
many divisions of metrical feet, are there?  Indicate a
division between every iamb ^ /:
When^ old' / age^ shall' / this^ gen' / -e^
ra' / -tion^ waste',

The rhythmic pattern falls into five
repetitions, or five feet, or pentameter.
Meter has two meanings. If someone
asks, "What is the meter of this poem?" the answer includes
both rhythm and feet. One answer might be, "The meter is trochaic trimeter." If someone
asks you, "How many feet are there in each line of this
poem?" it is correct to answer, "The meter is five
feet, or pentameter."

Check
each line to see if they are all the same. Poets may vary
the rhythm. Line 4 has a significant variation. Scanning beauty
according to its syllabic division causes a problem between
is and truth. Keats varied the stress this
way:  Beau^ ty' / is^ truth', / truth^ beau' / -ty^, - that' / is^
all'.

The line rhymes are
waste, woe, say'st, all and know. Some rhyme,
one doesn't. Assign a letter code to the words to see the rhyme scheme
emerge:

waste    a
woe    b 
say'st   
a
all    c
know    b

The
rhyme scheme is a b a c b. Each end word that does not
rhyme with one ahead of it gets a new letter designation. For example, dog,
see, cat, lap, down, crown would have a rhyme scheme of a b c d e e, with e e
forming a closing couplet (two rhymes lines).