The Power is equal to Voltage times Amperage. That is, P=VxA=Watts
For example, if you need to your cell phone 5V, 0.7A,, you will require 5x 0.7= 3.5Watts. Then, your wall/transformer should be of at least 3.5 Watts.
Also, the DC Power Plug or Jack should be connected of the form as it's showed in the Fig. #2 for drawing the terminal positive and negative.
It's important you realize that kind of unregulated transformer you are using for reading its specifications because the type of transformer must be with center positive for matching with the connection of the DC Power Plug or Jack of this project.
In contrary case, you should invert the connections of the DC Power Plug or Jack. That is, if you have an unregulated transformer with center negative, the terminal negative will be at center by having that invert the current connections for looking as in Fig. #4.
Let me show through a sign in the Fig. #4 that those connections aren't used in this project.
But, for this project is used an unregulated transformer of center positive (Fig. #3) that match with the connections propose and showed on the Fig. #2
Although the power jack looks of natural way in the picture showed after the fig.#4, in the last picture showed in this step we can note the identification of the connectos are A, B and C for connecting A and B as terminal negative and C corresponds to the terminal positive of our power jack male cited.