What must one who has masturbated, intentionally and unintentionally, during the daytime in Ramadan do to atone for their sin?

You committed a great sin by intentionally masturbating during the daytime in Ramadan, which invalidated your fast. Your sin was compounded by neglecting the obligatory ghusl (ritual bath) and sufficing with wudu (ablution).

Now, you must: 1. Perform sincere repentance to Allah Almighty. 2. Make up for the day you invalidated by masturbation. 3. Make up for all the prayers you performed before performing ghusl from janabah (major ritual impurity). 4. Abandon the forbidden habit of masturbation.

As for the other day when semen was discharged involuntarily, without intention, there is nothing upon you for it, otherwise you would have to make it up. The discharge of semen necessitates ghusl, and if it is discharged again before you perform ghusl for the first instance, one ghusl will suffice for both.