There is no "one-size-fits-all" tutorial for this.
Perhaps learn the basics first? Get a book that teaches C programming. That should get you started.
I've already gotten the speech about learning c. Thought I would ask about what to do since I have no idea how to connect the dots or where any supporting documentation for porting would be. I didn't see any good examples. But maybe there wouldn't be any since like you said there's no one size fits all tutorial?
The dots will connect once you learned how to code yourself. Then the process will be a lot clearer.
There is a Porter's handbook but the information in it will be totally useless if you don't know what any of it means.
There is no single way to do this. It all depends on the actual code you're dealing with. Sometimes it's easy, sometimes it's difficult and other's are close to impossible.
Jeez, you haven't set yourself much of a goal here, have you?
git clone https://github.com/samplesoftware/ cd into it and perform a make or configure or cmake or whatever software compilation control software is used. Pray it doesn't use meson. 
Im trying doing this with a program called i2p bote and when I cd into the directory and use make configure this is what I get...Jeez, you haven't set yourself much of a goal here, have you?
Source code is exactly what it says, the source code. It's the language of the compiler you're using. So for C, C++, C#, Objective-C each has its own source code syntax and language. If you have no knowledge of programing in C, then I suggest it's a poor language to learn concepts on. That's better done with a higher level language.
If you want to take some code off github, you can clone it to your local machine. So, the first thing to do is install GIT.
Once GIT's installed, go to the github repository and look for install instructions, a README.md or something similar. Any code worth running will have install instructions. Then you just click the CODE button on github, copy the URL and paste it onto your FreeBSD terminal, such as:
git clone https://github.com/samplesoftware/
This will pull in everything and create a sub-directory of the work. You will thencdinto it and perform a make or configure or cmake or whatever software compilation control software is used. Pray it doesn't use meson.
A post on Reddit suggests the original I2P-Bote project is abandonware, and it appears to be true.
make configure failed is because there is no Makefile with a configure target. In fact, there is no Makefile at all. You need to read the instructions.