I understand what assonance and consonance are, but I am having trouble finding this in each line of "Sonnet 18." Will you please help me?

Consonance occurs in the alliterative sh- that occurs in lines 1, 3, 4 and 5 (shall, shake, short, shines) and again in lines 9 and 11 with the repetition of "shall."

You can hear assonance in the long i sound in lines 5, 7, 8, and 9 (eye, shines,  sometime, declines, by, thy).