What You Need
Brawl 1.01 or 1.02 NTSC ISO [no links/asking where to get one, you need to do this on your own]
If You Have a 1.02 ISO
Download and extract this. Run xdeltaUI.exe. Select Revision 1.xdelta as the patch and your 1.02 Brawl ISO as the source file. Choose a name for your new 1.01 ISO as the output file. Then hit patch. It will appear that xDelta is frozen, but it is not. Wait for the patch to finish (could take several minutes), then verify the MD5 of your new ISO.
Idk how to explain how to do this. D:
Follow these instructions but use the patch from the Windows download.
Setting up Dolphin
If you followed the above guide for the Dolphin Build, then all your settings should be in place. To double check:
Turn on Enable Cheats in Options->Configure->General.
Right click your Brawl iso in Dolphin, click Properties->Gecko Codes and turn on the Global Brawl Netplay Settings cheat. This should be the only cheat on.
Playing on Netplay
Please follow the Netplay Guide
Here are some other reference guides in case you need any further help.