The most direct answer is: through his poetry. Donne self-consciously regards his verse as the instrument by which his love will be canonized.

By this, he means it will be both exalted into "sainthood" and made official, as biblical books and literary works have been approved and raised to such a level. It is an interesting concept in light of Donne's beginning the poem with a rather defensive reaction to those who would criticize him for his love. He asserts that his passion has implications of such universality that whatever people call him and his love, this is what they become: they are, one after the other, two flies, two tapers, the eagle and the dove, and the phoenix.

Yet the follow-up to this succession of iconic entities is what Donne seems to regard as the most valid extension of his love and his ego: poetry.

We can die by it, if not live by love,
And if unfit for tombs and hearse
Our legend be, it will be fit for verse.

The launching point for this metaphor is the verb to "die." English poets followed the symbolic use by the Italian poets of morire, to die, as associated with sexual climax. It gives Donne the opportunity for a conceit, in which he likens his poetry to a "well-wrought urn" holding the ashes of the dead, which, despite its small size, is at least as good as a "half-acre tomb."

The deeper point may be that Donne's poems, which are essentially miniatures, are as meaningful as any epic and will be just as effective as a means to canonize, or to "immortalize" (as Spenser said of himself and his mistress), his love. It's a standard trope in the history of literature that the writing itself is what causes its subject to be great. Donne and his mistress, through his poetry, will build in sonnets a house of "pretty rooms" (in Italian stanza means "room"). Therefore, the poetic form becomes a microcosm of a dwelling, but the lovers themselves become a microcosm of the world, whose "soul" is reflected in their eyes. (Donne has used this conceit about a world reflected in the eyes more than once.) This then becomes not merely a canonization but an apotheosis, in which his depth of feeling is so ideal and perfect that one must

Beg from above
A pattern of your love!