I've updated my answer to address your follow-up question. I would have sent you a message, but your profile appears as "hidden."
I think you've marked the feet correctly in those four lines that you quote from the poem "The Nymph's Reply to the Shepherd." Each foot contains two syllables; the first of those syllables is unstressed and the second is stressed. Put what you're calling the breve over the unstressed syllables and the ictus over the stressed syllables.
The first line thus will look like this:
˘ ˊ | ˘ ˊ | ˘ ˊ | ˘ ˊ
But could youth last, and love still breed,
(NOTE: I can't seem to get the items to line up correctly. The breve goes over "but," "youth," "and," and "still." The ictus goes over "could," "last," "love," and "breed.")
The trick to finding the unstressed and stressed syllables is to say the line in a very natural way several times over, listening each time for the beats or stresses. When speaking normally, English speakers tend to use an iambic meter (the same meter that is used in this poem), so the alternation of unstressed and stressed syllables is the first thing that you might listen for.
NOTE: It doesn't matter exactly what letter the accent goes over. The accent should go over the middle of that syllable. Some syllables have just one letter, some have several letters. "Telephone" has three syllables: tel - e - phone. The first and third syllables get some stress; the middle syllable does not. The accents would go over the "e" in "tel," over the "e," and over the "o" in "phone."