First Cream Soda- Carbonation Problems

I and a friend last night brewed a 5 gallon batch of Gnome Cream Soda. We followed the instructions on the website ---> Root Beer Recipe & CO2

When it was time to carbonate it that's when we hit trouble. The instructions just say to carbonate at 35psi. We've been brewing beer up to this point and my friend just assumed to carbonate like beer. So we filled it with 35 psi, turned the corny tank upside down and shook it like we would with a batch of beer.

He tried it today and he said it was really flat. The instructions say to repeat as necessary but he doesn't want to do anything until we find out what where doing wrong I guess.

Our main questions is do you carbonate soda the same way you carbonate beer? What is the difference if there is any?

Shouldn't be a difference there. The keg is at 35psi today (no leaks etc?).

You do need to make sure you have either a longer hose or a mixer inside the liquid out dipstick for resistance so that the carbonation stays in the liquid when it's poured.

I carbonate at a higher PSI, typically 45-50psi.

What tube lengths were you using, and how cold was the drink when you carbonated it?

I use 30 psi for my soda, at 40 degrees, with 25' serving lines (3/16"). It takes a while to carbonate it without shaking, but if you shake make sure the keg is cold first.