How do I write sonnets?

Read about the sonnet: Sonnet – Wikipedia

Read lots of sonnets.

Get comfortable with writing in metre. Start with blank verse.

Get a rhyming dictionary.

Pick a sonnet scheme. The sonnet scheme determines the structure of the argument you’re going to be making in the sonnet.

• In the Italian sonnet (the Petrarchan), the octet (first eight lines) make one argument, and the sestet (the last six lines) makes a new argument: there’s a volta, a twist, a turn, at the start of the sestet. The rhyme binds the octet tightly together, and the sestet tightly together. The octet breaks naturally into two quatrains, and the sestet into two tercets. So there’s a finely poised thesis/antithesis, and a contrast in rhyming structure between the two.
• In the English sonnet (the Shakespearean), you have three quatrains, each with their own rhyme, and a final couplet; so there is less of a sense of the quatrains being bound together. (In the Spenserian sonnet, the quatrain rhymes are related.) Before Shakespeare the volta was still at the start of the third quatrain (same place as in the Petrarchan). In Shakespeare the volta is in the final couplet, as a badoom-tish. Each quatrain advances the argument, and the couplet is either a summary, or a Woah Didn’t See That Coming!

Why yes. You’re making an argument. That’s a critical thing about the sonnet, whether in Italian or English form. It’s not a spontaneous outpouring of inspiration: it’s art. The scheme corresponds to an ordered presentation of a thesis and antithesis. Maybe with a synthesis in the final couplet or the sestet.