How to compile with headers and a create a Makefile - C

I am trying to compile and create a Makefile with my source code organized as per the following structure:

|-- build|-- include| \-- functions.h\-- src|-- functions.c\-- main.c

The code written there has been tested on an IDE, so I know it works. I just need to compile it but I don't know how to execute 2 files with extensions .c and the .h file. ALSO, I need to create a makefile for this.

How?