How to scan Sonnet 73.
Mark the syllables, separate the feet with short verticles lines, and indicate the rhyme scheme.
That time of year thou mayst in me behold
When yellow leaves, or none, or few, do hang
Upon those boughs which shake against the cold,
Bare ruined choirs where late the sweet birds sang.
I love scanning poems. Once you get the system down, it's not hard to do, and sometimes the scanning will show you something that you might otherwise overlook (such as how stresses can bump up against each other just like the wagons of a train in Claude McKay's famous sonnet from the 1920s, "On the Road").
1. Mark the syllables: Simply count out the syllables (the units of sounds, usually a combination of a consonant and vowel) on your fingers.
That - time - of - year - thou - mayst - in - me - be- hold
Thus, the first line of the quatrain that you give in your question seems to contain 10 syllables. "Mayst" could possibly be pronounced "may - est," in two syllables, so I would recommend checking the other lines to see if 10 syllables are indeed the regular pattern. They are.
2. Separate the feet with short verticles lines: To complete this step, you need to understand what stresses and metric feet are. Stresses are natural emphases when you read the words aloud.
That - TIME - of - YEAR - thou - MAYST - in - ME - be- HOLD
You can see that the first line that you quote has alternating stressed and unstressed syllables. This pattern is a clue that a certain metric foot -- called iambic -- is being used in this poem. Iambs are units of two syllables; the first syllable isn't stressed and the second syllable is. Iambic meter is the natural rhythm of English and is the meter used in this sonnet. Here's the first line that you quote with the lines added:
That time | of year | thou mayst | in me | behold
Again, once you've found a pattern in one line, it's wise to double check to see if that pattern is indeed used throughout the poem.
3. Indicate the rhyme scheme: The rhyme scheme is found in the final words of each line. Assign each final sound a letter, starting with A; assign a new letter every time a new sound is added but use the prexisting letters every time a sound is reused.
behold = A
hang = B
cold = A
sang = B
Thus, this stanza has an ABAB rhyme scheme.
If necessary, you'll need to repeat this process for all of the other sections of the poem. Have fun!