This is basically a simple version of this guide here >> https://help.ubuntu.com/community/Installation/QuickNetboot.
Ubuntu PXE Netbook Images can be found here.
You want the link for the file named /netboot.tar.gz copy the link if you aren't installing 16.04 Server. That is the only difference to change distro's for this guide.
This assumes a tftp root of /srv/tftp.
You should successfully download pxelinux.0 over tftp. This is a great way to find out if your clients will get that far!
DNSMasq Config :
Read the linked guide for troubleshooting steps, they have some IPTable rules that you may need to implement!