Ghusl e Jumma is like other ghusls with no difference. Therefore first the head and neck has to be washed, then the right side then the left. It's time is before Namaz e Zuhr. The closer it is to dhuhr, the greater the reward. After Namaz e dhuhr, ghusl can be done as well with the intention of Ma fi al- dhimma (to fulfill whatever one's legal obligation happens to be). The ghusl can be done on a Saturday as well with the intention of qadha.
In regards to niyyat, during wudhu, ghusl, namaz the niyyat is just that you know what you are doing. It it not necessary to say it. Hence if I were to be asked you what am I doing, I would for example say wudhu or ghusl which shows I had the intention of wudhu and started this action. This is enough for the start of an action. That is I know what I'm doing. It is not necessary to say the niyyat in words such as I'm doing so and so ghusl. As long as I know what I'm doing, it's enough.