After you get your Linux installation up and running, please continue the following process to build your OpenWRT.

• Login to Ubuntu and open (press Ctrl+Alt+T) the Terminal as shown in figure below:
  

• Downloads the package lists from the repositories
  

  sudo apt-get update

• Download & install necessary packages for the cross-compilation process
  

  sudo apt-get install subversion build-essential libncurses5-dev zlib1g-dev gawk git ccache gettext libssl-dev xsltproc sudo apt-get install unzip python wget

• OpenWrt source repository downloads, I am using the release version of Barrier Breaker 14.07.
  

  git clone git://git.openwrt.org/14.07/openwrt.git

• Download and install all available "feeds". Depend on your internet connection, it takes some time to complete this process.
  

  cd openwrt ./scripts/feeds update -a ./scripts/feeds install -a

• Make OpenWrt Buildroot check for missing packages on your build-system using one of the following commands;
  

  make defconfig make prereq make menuconfig

• After a few seconds you should see the main screen for configuring OpenWRT. In this tutorial, I'm selecting TP-Link TL-MR3020.
  
  
  

• Exit the menu system when finished going through all the categories of software. Make sure to confirm that you want to save your OpenWRT configuration.
  

• Optional: You can also modify the kernel config for the selected target system. Depend on your internet connection, it takes some time to complete this process.
  

  make kernel_menuconfig

• After the process is completed, you should see the kernel configuration menu.
  

• Exit the menu system when finished going through all the categories of software. Make sure to confirm that you want to save your OpenWRT configuration.
• With all of your packages selected and your configuration saved, you’re ready to build your firmware. It takes one to several hours to complete this build process. 
  

  make

  If you want 4 parallel jobs from make
  

  make -j 4

  or watch it do its thing
  

  make V=s

• The result firmware image is generated in the Home/openwrt/bin/ folder.
  

• Flash the factory.bin to your router.
• You may want to get a clean build after running the "make" command. If you do a make distclean, buildroot will delete your .config file.
  

  make distclean