Use the correct screws, i don't have it on the top of my head someone else can fill in there.

REUSE screws

Use a small screwdriver (flathead) to scrape off the "green stuff", as pictured, be carefull tho not to go mental on it, when you open your N900 and look at the circuit then you will see where you can scrape of.
Scrape of on both sides, just not much on the "small" size, again when you open it up you will see what i am trying to explain.

Again this is a very simple fix and you really really have to mess it up bad to ruin anything.

As for the soldering, warm up the iron of the USB so you are sure that solder will stick to it and then solder it to the BASE (where you scraped off the green stuff, this is right up against where the USB is soldered to the board with a very little contact patch (hence why it comes of "easy").

Hope this has helped, please report back after you done it, also if you can take pictures/video of the fix please link post and i will add to post #1/2.