This is a super easy, cheap project that any amateur with some gloves, rags, and sandpaper can complete. I made this desk for my girlfriend as a gift, and she loves it. I'm going to give you a quick overview. There are some other resources online that might help you but for me, the best way of going about this project is just to jump in and improvise where need be!
List of Stuff: Table Top / Lumber / Brackets / Stain / Stain Applicator (T-Shirt Rags) / Latex Gloves / Sandpaper / Steel Wool / Hammer + Nails (Optional) / Beer
Step 1: Buy a Table Top
I bought my table top at IKEA. VIKA FURUSKOG. $40. It was the perfect size. If you wan't something longer or wider, the world is your oyster. I've seem some incarnations where people used old doors as table tops. They looked awesome, but this would require a lot more work.
Note: I didn't do this step first and it made the project harder. Once you have your table top, coming up with the dimensions for your other pieces will be much easier!
Step 2: Buy Some Lumber
As the title suggests, your next step should be to go to your local hardware store and purchase the wood that will constitute the legs and cross braces of your desk. Don't be shy. If you feel like this is too much work, there are plenty of already-made sawhorse you can buy, you lazy bum!
What kind of wood: A few 2x4s should do you (I bought three at 12' which made for 8 legs, 2 braces, and some extras). Note: Having extra pieces is good in case you screw up. Don't worry about what kind of lumber but Don't Buy Treated Wood. If you're like most people, you'll probably be working at your desk indoors, so there's no need to buy weather-treated wood.
How to get your wood cut: Most hardware stores (Home Depot, Lowes, Ace) have a saw to cut your lumber for you. Just grab the nearest employee and ask if they can operate it for you. Or if you're like me, stand by it until someone takes pity on you. Once you find a saw operator, there going to ask you what length you want the pieces cut. For my desk, I had the legs cut at 19'' and the braces cut at around 23". If you think 18.75" and 22' seems better, by all means. Just remember that the sawhorse brackets will add a few inches in height.
All in all if you do this on the cheap without buying the premium lumber, this step should only run you around $20.
Step 3: Buy Sawhorse Brackets
They're about $20 for two pairs (four brackets) which is all you need. Look in the hardware aisle or ask an employee.
Step 4: Buy Wood Stain + Other Stuff
This is the step where you can really make your desk your own. There are so many different kinds and colors of stain that it can be a bit overwhelming at first. I used a regular polyurethane stain which are kind of two-for-ones, i.e. stain and sealant. All the sealant does is give the wood some durability. But the sealant will also give your a desk a slightly shiny finish, so you'll want to weigh your options. If you're looking for a flatter, matte finish, you might want to forgo the sealant/stain combination. Around $25.
Other Stuff: Fine Sandpaper / Gloves / T-Shirt Rags / Steel Wool.
Note: If you have old t-shirts around, go ahead and rip them up and use them. Most people prefer white t-shirts because its easier to gauge how much stain is on the rag.
Step 5: Get Sandin'
This will be the most annoying part of the project. I recommend using an electric sander. I owned one beforehand but if you feel like you can swing the extra change, Ryobi makes a cheap, durable one.
You'll want pay particular attention to the ends. Sanding them down will make it easier for them to slip into the brackets and it lessnes the chance you'll get stuck by a splinter. Once your done, wipe off any excess dust.
Step 6: Get Stainin'
I recommend doing this outside. If you're using a table, be sure to set some newspaper down as this tends to get messy. Also, be sure to wear latex gloves.
Depending on how dark you want the desk to look, you'll want to apply at least two coats. Make sure you block off a good amount of time for this step. After you apply a coat, not only will you want to wipe off any excess stain a few minutes after its applied, but you'll want to let each piece sit and dry for an hour or so. Once they're dry, take your steel wool and lightly scuff each piece. This allows for more stain to penetrate the wood on your next coat.
Note: When wiping excess stain, be sure to go with the grain of the wood. But if you don't, its not a big deal. There will be some streaks but the desk shouldn't look perfect.
Step 7: Assemble!
Once your happy with the color and the pieces are dry, go ahead and assemble the two sawhorses.
Disclaimer: You'll notice that each bracket has holes for nails. You don't need nails. Other people have made perfectly fine sawhorse desks without them. But In my opinion, nailing the brackets in makes the desk feel much sturdier. If you decide to get a hammer and nails, use your best judgement. Standard, 16-penny nails and a small hammer should do the job. Also, try to make sure you're setting your table up on level ground.
When the sawhorses are done, set the table top ... on top.
Step 8: You're Done!
Congrats. You're a carpenter. You can now tell you're friends you have something in common with Jesus.
Be proud. At this point, you should feel pretty good. If not, sorry. For you happy people, enjoy a celebratory beverage as you pat your handy-self on the back and examine your work.