Mussaf time begins from day break until the end of the day. However, you should not daaven it before Shacharis. L’chatchila, you should say Mussaf immediately following Shacharis. If you delay it until after the seventh hour, you are considered negligent. (ShA 1 – MB 5)
There is duchaning during Mussaf. (ShA 1)
Once you daaven Shacharis you are obligated to say Kiddush before you eat. Are you allowed to say Kiddush and eat before Mussaf? No, not a seuda. A snack is allowed: A k’baytza of bread (no more), a cup of wine, a k’baytza of Mezonos. Fruit, even a lot of it. (ShA 3 – MB 7, 8, 9)
There is an opinion that you are allowed to have a seuda before Mussaf. You can rely upon this if you are feeling weak. Eat even more than a k’baytza until you are feeling better. (MB 9 – ShaH 9)
If you are feeling weak and you don’t have any mashke. for Kiddush, before Mussaf you are allowed to eat some fruit and some Mezonos even without Kiddush. (MB 9)
Once Mincha Gedola arrives and you still have not daavened Mussaf, daaven Mincha first. This applies only to an individual but a congregation should daaven Mussaf first so it does not lead to confusion. (ShA 4)
If there is not enough time before shkia for both Mincha and Mussaf, you can daaven Mussaf and then two times Maariv as a tashlumin for Mincha. (MB 13)