This is super easy, It only takes about 20 min at the most to actually assemble. Optional painting might take longer.

All you need is:

Tools:

A drill/driver

Supplies:

A piece of counter top that is the size you want for your coffee table. (I found a piece at a Habitat For Humanity Restore for $5)
Four leg mounting brackets (I got mine for about $2.50 each)
Four legs with screw bolts embedded of an appropriate height for your needs (I paid $6 each for these)

Optional items:

Painting supplies to decorate the top as you wish

Step 1: Step 1: Finding the Right Materials

Finding a cheap and usable piece of counter top is the key to this easy table. I used a Habitat for Humanity ReStore. They sell reclaimed building materials. I found a 25 inch by 62 inch piece of used counter top that was on sale for only $5!!!

Step 2: Step 2: Decorate the Counter Top (Optional)

We decided to paint the counter top. This step actually took up the most time but it's completely optional.

Step 3: Step 3: Add the Mounting Brackets

There are a few different types of mounting brackets and even a few kits with legs included. You should choose whatever fits your style and budget.

To attach the brackets just use a drill or driver to screw in the included mounting screws. For this hardware there were five screws per bracket.

I attached one bracket in each corner at approximately the same spot so that each leg is symmetrical with the other legs. This will make your coffee table more stable.

Step 4: Step 4: Attach the Legs

The legs should just screw into the brackets.

If you want to paint the legs, I didn't, you should paint them before you screw them on.

Flip your new coffee table over and make sure it is level and stable. Adjust as needed.

Once you've got it the way you like it consider adding some plastic furniture gliding feet to the bottom as we discovered the legs don't slide well on carpet. Of course that's optional.

Step 5: Step 5: Optional Things, Otherwise Enjoy!

If you went with the mounting brackets you may have already figured out that this can make almost any height of table depending on the legs you choose. We choose 15 inch legs initially but swapped them out for 21 inch legs after a week or so. If you used taller legs you could make a counter height table instead of a coffee table.

Enjoy your new table!