advice please

My MOTHER-IN-LAW want's me to pressure was her deck, Oh boy! This is not my side of the business, so some advice would be helpful, what cleaner are needed if any. I have 3000 PSI at 5 gallons per, and a surface cleaner, If anyone could give me a run through of the procedure I would appreciate it greatly.
Is it as simple as pressure washing it evenly, and sealing it? What problems should I look out for

What is on the deck currently?? No sealer/stain? A clear sealer? Or one with color?

If there is something on it, ask if they know what brand it is.

Ditto to what Mike Hughes said about the cleaners, etc. Also, I would not use a surface cleaner on the deck. I've never done it, but I believe it would be very difficult not to leave swirl marks.

I think it has nothing on it. I looked at it today, they need to replace s few of the boards, and want them to look somewhat uniform.