Start by asking your instructor for any requirements that need to be included in your assignment. If specific techniques or forms of construction need to be addressed, be certain to cover those topics.
Both poems are presented as conversation or thoughts of the speaker, using very common but expressive language. In both cases, the voice of the narrator closely parallels many of the thoughts and characteristics that have been attributed to Frost himself.
Both raise questions about the activities being described in the poems. "Before I built a wall I'd ask to know what I was walling in or walling out and to whom I was like to give offence" expresses bemusement and skepticism about the worth of the activity; from a different viewpoint, so does "The woods are lovely, dark and deep, but I have promises to keep, and miles to go before I sleep."
Imagery is used with great effect in both poems, with nature and humanity's interaction with it playing a prominent role. One poem has a regular rhythmic pattern and uses rhyme to propel the reading, while the other is open verse.
I would compare or contrast the theme of each poem to begin with. There is a definite theme to each poem and to delineate these is a good starting point for your compare/contrast paper.
For example, the final line (a repeat of the line before) of 'Stopping By Woods on a Snowy Evening' hints at this particular poem's theme:
"And miles to go before I sleep."
Next, I would compare or contrast each poem's Point of View (POV). The poem may be first person POV (I), second person POV (You), or third person POV (We). Strive to understand why the writer chose the particular POV for a particular poem.
I would also compare contrast the rhyming scheme of each poem, the stanza organization, as well as the meter of each poem (Blank Verse or otherwise).
In addition, I would compare or contrast the language used in each poem. Is one poem more formal in tone, while another is more conversational in tone with slang and local phrases and such. Again, ponder why the writer chose the style of language he chose for each poem.