The first step is register for an account. After that there are two simple steps.
The preferred method to generate a trace is to use this stored procedure. It generates the bare minimum trace required. It also limits the trace to 10MB which is the largest trace file you can currently upload. You can also review more detailed instructions on running the stored procedure
You can also use Profiler to generate your own trace. Please be sure to include the RPC:Completed and SQL:BatchCompleted events and the TextData, CPU, Reads, Writes and Duration columns. Please limit the trace file to 20MB or less. You should get roughly 25,000 events per 10MB trace using this format.
After you have registered you'll see a link above to Upload a Trace File.