freebsd kernel module - “make” command

Hello, I am a newbie in FreeBSD. I installed FreeBSD 11 on VMware. I want to write first FreeBSD kernel module. I find this link: How to write a FreeBSD Kernel Module

But in step3, after make command I get:
unable to locate the kernel source tree. Set SYSDIR to override.

I searched it a lot, and I found that I should install kernel sources. But how can I do it? I want step by step guidance.
Please help me.
You can checkout the sources with either SVN or Git:

svnlite co /usr/src


git clone --single-branch -b releng/11.0 /usr/src