This is a standing desk for ergonomic computer use, made completely from recycled pallet and construction wood. Using recycled wood is, of course, completely optional.

Step 1: Materials

Construction Materials

Tabletop board
8-10 sturdy boards for legs and supports (see pictures)
At least 30 6 cm (roughly 2 1/4 in) wood screws

Tools

Tape measure
Electric drill, drill bits, and driver bits
Table saw
Circular saw (recommended, jigsaw is an adequate substitute)
3 Clamps
Square
Random orbit sander (for optional finishing)

Optional Note On Dimensions:

Since this is a recycled wood project, the size of everyone's piece will probably differ slightly. Don't worry! In case it helps, the dimensions of the pieces used in my build are:

Tabletop board 2.5 x 95 x 80 cm (1.0 x 37.5 x 31 1/2 in)
4 legs 4.0 x 7.5 x 95 cm (roughly 1 1/2 x 3.0 x 37 1/2 in)
2 bottom leg supports 4.0 x 7.5 x 95 cm (roughly 1 1/2 x 3.0 x 37 1/2 in)
2 top leg supports 4.0 x 7.5 x 73 cm (roughly 1 1/2 x 3.0 x 28 3/4 in)
2 lateral supports (optional) 4.0 x 7.5 x 73 cm (roughly 1 1/2 x 3.0 x 28 3/4 in)

The legs and supports are all made from the same type of pallet piece, shown with an arrow in the last picture of this step. It has a 4.0 x 7.5 cm cross-section and is 100 cm long (roughly 1 1/2 x 3.0 in cross section and 39 3/8 in length). A typical 2 x 4 style board would be a fine substitute.

The tabletop board was being thrown out at a construction site near me. It was a great piece in excellent condition—a lucky find! A piece of 3/4 in plywood would be a great substitute.

Step 2: Cut Tabletop to Size

Cut the tabletop board to the size you prefer for your desk. Mine is 95 x 80 cm (1.0 x 37.5 x 31 1/2 in). A circular saw works best here, but a jigsaw would be a fine substitue. Clamp down a fence to make sure your cuts are square (see picture).

Step 3: Cut the Leg Pieces

At the end of this step, you should have 4 leg pieces of proper length, cut with the angles shown in the first picture. First, decide on a leg length (instructions follow). Once you know the leg length, cut the legs to length, using an angle of 10 degrees for the slant. Make sure you get the orientations of the angles right for the leg pieces. Study the first picture in this step carefully if you are unsure. Use a miter saw or table saw with a miter sled to get the slant (picture 2).

TL;DR Leg Length Instructions

The leg length decides the height of your table. When cutting the legs to length, take into account the height you want the tabletop to be, the extra height from the thickness of your tabletop, and the 10 degree slant angle of the legs. When you know the length you want, cut the 4 leg pieces to length. Using a miter saw or table saw with a miter sled to get a 10 degree slant angle on both sides.

Detailed Leg Length Instructions:

The length of the legs depends on how high you want the tabletop to be, which depends on:

Your height and body shape
The slant angle of the table legs
The thickness of your tabletop board

My suggestions are:

For use with a keyboard, make the final tabletop height about the height of your waistline (where you wear your belt). For reference, I am 175 cm / 5' 9" and my tabletop height is 98 cm or 38 1/2 in.
Use a slant of 10 degrees. This angle is small enough that you can skip trigonometric calculations. If you don't mind math, see the note below.
Remember to take into the account the thickness of your tabletop board! This will add a few cm to the final height.

Simple formula: leg_length = desired_tabletop_height - tabletop_board_thickness + 1 cm or 0.5 in

The extra centimeter helps correct for the effect of the slant.

If you don't mind basic trigonometry (see note below), use the formula:

leg_length = (desired_tabletop_height - tabletop_board_thickness) / cosine(slant_angle)

Math Note: Don't worry, this step only adds a centimeter or so of precision and can be ignored. Because the legs are on a diagonal, the length you cut them to and the final height are different. The height is the leg length times the cosine of the slant angle, so the leg length should be the desired height divided by the cosine of the slant angle. Note that for the suggested slant angle of 10 degrees, cos(10) ≈ 0.985, which is so close to 1.0 that the extra precision is almost negligible.

From above: cut the legs to length, using an angle of 10 degrees for the slant. Make sure you get the orientations of the angles right for the leg pieces. Study the first picture in this step carefully if you are unsure. Use a miter saw or table saw with a miter sled to get the slant (picture 2).

Step 4: Assembling Leg Pieces Part 1

At the end of this step, you should have two half-assembled leg pieces that look like the first image.

1. Cut the 2 bottom side supports to the correct length and angle.

The angles should be oriented like in the second picture. Use the same angle you used for the slant angle in the previous step.
The length should be long enough to fit somewhere across the bottom fourth of two leg pieces from the previous step (see first and third picture). The exact length is not critical.

2. Make sure the leg pieces are square like in the third picture, then screw the support piece onto the tops of the legs.

Note: My screws weren't long enough for the thickness of my boards, so I drilled a small hole in each support piece so the screws would reach into the legs. This is shown in the fourth picture, and you can skip this step if your screws are long enough.

Step 5: Assembling Leg Pieces Part 2

At the end of this step, you should have two fully assembled leg pieces that look like the first image.

1. Cut the 2 top side supports to the correct length and angle.

The angles should be oriented like in the second picture. Use the same angle as in the previous step.
The length should fit exactly across the top of two leg pieces from the previous step (see third picture).

2. Ensure everything is square, clamp the assembly down so it won't move (see fourth picture), and screw the support piece across the tops of the legs.

Step 6: Attaching Legs to Tabletop

Don't copy my mistake! Before assembly, make sure the finished desk will fit through your workshop door. If not, carry the components to the final location and assemble the desk there. I had to end up taking the desk apart to get it out of the workshop ;)

The assembly is easiest upside down.

Clamp the leg pieces to the bottom of the tabletop board like in picture 1. Note that the legs are slightly offset from the edges of the table.
Screw the legs onto the tabletop. I suggest driving the screws diagonally for a better hold. If necessary, drill a pilot hole to accommodate the screw (picture 2). If you have a pocket screw machine, use it here.

Step 7: Optional: Add Lateral Supports

For extra lateral support, you can add boards between the leg pieces like shown in the picture. A similar piece is on the other side; it is just not visible in the picture. This will help prevent side-to-side wobbling.

Cut the board to a length that just fits between the legs and screw (or glue) it in place. I used diagonally-driven screws.

Step 8: Finalize Set-Up and Enjoy!

I used an old TV stand to raise my monitor to eye level. Any manner of things could be used instead (e.g. books, boxes, a step-stool, etc...). I use a USB keyboard and mouse, and connect a laptop or computer to the monitor with an HDMI cable (not shown).

Enjoy!