It's very simple.
To start with, I go op shopping. See, this is fun already :-) You're looking for timber frames in good nick. I look for timber because they're easier to paint and sand back...to customise. Often I'll pay $2 each for decent sized frames.
Chances are you have some acrylic paint around the house, but if you don't, then try the mis-tint section of your local hardware store. I've often found pretty colours at $1 or $2 for a 250ml sample pot. Even children's acrylic paints will work.
Now, I'm very sorry but weebly seems to refuse to let me load my own images on the blog most of the time. Not sure why. So though I took some pics of some homemade examples, I can't load them! So I have example pics from elsewhere.
Homemade chalkboard paint is this: 1 cup acrylic paint to 2Tbsp extra fine tile grout. Then it needs to be blended VERY well. A stab blender does the job fairly well, but you've still got to look out for little lumps.
More recently I've just taken the timber backing out of the frame (discarded the glass), painted it with black children's acrylic, let it dry, and then wiped a dusty duster over it to prep it for writing on. And like previously mentioned, this has worked fine.
For the frame itself, there are many options. But a quick and effective choice, is to paint the frame with your chosen colour, let it dry, and then take some sandpaper and rub back some of the edges to get that rustic farmhouse effect.
My sister uses her homemade chalk board for her shopping list. I have several, mostly with scriptures written on them throughout the house. They're also great as 'slates' for learner writers to practice.
Some people use them to display the daily/weekly menu for the household...so many options.