Aligning shapes
When you place shapes on to a drawing page, it is not always easy or possible to line them up accurately. In Visio, there are tools that allow you to automatically align shapes along their edge or centre point, both vertically and horizontally.
It is important how, and in what order, you select your shapes before you try to align them. All shapes that you select will be aligned with the Primary shape (the one with the thick pink border around the outside when you have more than one selected).
If you select your shapes individually using the Ctrl key, then you will need to ensure that you select the primary shape first, followed by all others.
If you make your selection by dragging a box around them or using Ctrl + A the primary shape will be the one that was placed on to the drawing page first. This may not be necessarily the one you want to align to.
To align go to Shape > Align…

You will have the choice as to whether you want to align horizontally or vertically and if you want to align down the left, centre or right side of the shape. If you are using connectors to join them (as in a flow chart), you will need to align throught the centre or middle.
Space shapes evenly (distribution)
Visio allows you to automatically space shapes evenly, using Distribution. You can only perform this action with a minimum of 3 shapes. Distribution keeps the 2 outer shapes in place and moves the ones in between to create an equal space between all of the shapes.
To distribute go to Shape > Distribute…

You will have the choice as to whether you want to distribute horizontally or vertically and if you want to measure the space equally from the outside or the midpoint of the shape.
It is recommended that shapes are aligned prior to distribution and that the 2 outer shapes are in the correct position.