If I understand your question correctly, you are to relate the opening stanzas to the subject or theme of the poem and show how the opening connects to the poem's ending. The first two stanzas form a simile. The first word of the poem is "as." Thus, the first stanza forms one part of a comparison. The two lovers parting is compared to virtuous men dying. When virtuous men die, they meet their deaths without protest, so imperceptibly that observers cannot even detect the moment of death. They are able to die so quietly because they have faith that because they have lived good lives they will go to heaven, that their deaths are not truly an end to life but a passage to another. The speaker of Donne's poem tells his lover that their separation should be likewise. Because they have faith in each other, their love is on a spiritual level, their love will not die when they are separated. Just as virtuous men do not fear death, the lovers should not fear separation. This then is the scene of the opening of the poem. We can imagine here the speaker speaking in a gentle tone trying to console his beloved who is anxious about his departing.
The final conceit regarding the compass is a stunning image that demonstrates in a visual way the connection that the speaker is describing between the two lovers. While the two legs of the compass seem to be separate on an earthly level, just as the two lovers seem to be separate beings, they are in actuality one--they are joined on a spiritual level, just as the two legs of the compass are joined above. Thus, even though one leg may venture away from the other--"far doth roam" --their love will never endure a "breach, but an expansion." The speaker shows that their love will endure any type of separation and that if the beloved stays "firm," their love will remain perfect, steadfast, and eternal.