Get out the microscope, because we’re going through this poem line-by-line.
In the stormy east-wind straining,
The pale yellow woods were waning,
The broad stream in his banks complaining,
Heavily the low sky raining
Over towered Camelot;
- The weather lets us know that things are all messed up. There's a stormy wind, the leaves are yellow and fading ("waning"). Even the river "complains" and the sky is low and heavy with rain above Camelot. The outside world reflects the Lady's sad situation.
Down she came and found a boat
Beneath a willow left afloat,
And round about the prow she wrote
The Lady of Shalott.
- Now the Lady does what pretty much everyone does when they feel bad: she goes and finds a boat and writes her name on it. Actually we're not sure why she does this, but it does make her easier to identify later in the poem.
And down the river's dim expanse,
Like some bold seër in a trance
Seeing all his own mischance--
With a glassy countenance
Did she look to Camelot.
- She doesn't get into the boat right away. Instead she hangs out for a moment and looks down the river.
- The speaker compares her to a seer (someone who can see the future) having a vision of his bad luck (mischance) in the future. She has a glazed expression ("glassy countenance") on her face. The Lady can sense already that she's doomed.