A very common problem is precisely aligning a texture to a face. For example, I have the following photo of a wall that I want to "straighten" and apply to a rectangle:
In order to make it clearer for this tutorial, I've added a red outline of the particular area I want to use.
Starting with a simple rectangle, I import the image as a texture and apply it to the rectangle face. Selecting the rectangle, I right-click to select the Texture >> Position option. This causes the colored push pins to appear. Right-clicking again on the image, note that the Fixed Pins option is checked.
The next step is to move the pins to the corners of the red rectangle. In Fixed Pins mode, click once on a pin and then it will "float" with the cursor until you locate it and pin it to the new location with a second click. Do the same for the other three pins. For this part, the order is not important.
The final step is to click (hold) and drag the pins to the corners of the original rectangle. The order in which you do this is important. Starting with the red pin, click and drag it to the lower left corner of the rectangle. Next, do the same with the green pin and move it to the lower right corner. Then move the blue pin to the upper left corner of the rectangle. Finally, click and drag the yellow pin to the upper right corner. Click outside the rectangle to exit the position mode.
This YouTube video shows the steps outlined above.