You should have already be able to use your Netduino already. Which means you have already downloaded and installed everything from http://netduino.com/downloads/ . The most important (for this tutorial) tool inside is MFDeploy.exe, which is found in "Microsoft .NET Micro Framework\v4.2\Tools". (alternatively, you can get MFDeploy from http://www.microsoft.com/en-ca/download/details.aspx?id=5475 directly)
Then you need to follow this guide first: http://wiki.netduino.com/Building-Netduino-Plus-2-Firmware-4-2-2-0-with-Yagarto-GCC-4-6.ashx?HL=gcc , but after the build finishes with no errors, you do not need to generate the DFU file.
(note: this is extremely time consuming, the downloads are huge, the installations are huge, the compile time takes 5 minutes on a high end computer, and the error messages might be confusing to the beginner. I don't mind if you skip these steps and just flash my precompiled firmware instead)
If you were able to complete this, there will be a folder that looks like portingkit\BuildOutput\THUMB2\GCC4.6\le\FLASH\release\NetduinoPlus2\bin\tinyclr.hex, and inside there will be some files (we need ER_CONFIG and ER_FLASH). Plug in your Netduino, and run MFDeploy.exe . See the screenshots below for the next steps.