Unwrap a cube

A cube is the simplest thing to unwrap: each of its faces is flat, so they all lie flat with no stretch at all. It can come apart as six squares, or stay in one piece as a cross.

Six squares, in one click

1. Open File and, under Examples, with Pre-unwrapped unticked as it is to begin with, click the cube under Primitives. It opens bare, and the UV Layout says it has no UVs yet.
2. Click Generate UVs. Every edge of a cube is sharp, so each face comes away as a square of its own, and the six are packed into the square.

One piece, as a cross

To keep the faces joined, cut only the seven edges a cross needs.

1. Open the bare cube again, and press 2 for Edges mode.
2. In the 3D View, click an edge of the top face, then Shift+click two more: every edge of the top but the one along the side facing you.
3. Drag to turn the model, and Shift+click the bottom face's edges the same way: every one but the one along that same side.
4. Shift+click one upright edge at the back.
5. Click Split: the seven edges turn red, as seams.
6. Click Flatten. The four sides lie in a strip, with the top and bottom either side of the one they're joined to: a cross.

Check it

Pick Checker in the Textures section's Preview. Flatten and Generate UVs turn on Textures in the 3D View, so the pattern shows on the model too: its squares come out square, and the same size on every face. See Color and Checker patterns.