Consider what strength you want to work at. For a quick blurring of part of an image, where quality isn't the most important thing, I suggest the default setting of 100%. Also, see the next step and work on a separate layer. You can then adjust the opacity of that layer, if you wanted, for a lower amount of blurring.

I recommend that you don't blur directly onto your image. I suggest you add a new empty layer and use the Blur tool on that layer. To enable you to do that, you will need to select Sample All Layers.

In this shot of golfer Rory McIlroy, I will blur the spectator and background behind him, so he stands out more.

The next shot is a close-up, so you can see detail more clearly.

Use a larger brush for the general areas, but reduce brush size where more accuracy is required, such as around the golfer, shown in the next step.

Use a small brush for more accuracy around things you don't want to blur.

Here is the blurred version.

It's that simple. Select the Blur tool and paint with it, though I do recommend doing so on a separate layer, as described in Step 3.

I will blur the buildings in the background, to make the cannon stand out.

To see detail clearly, I'll work on a section of the image.

Add a new empty layer.

On the Options bar, ensure All Layers is ticked.

Working on a separate layer gives you the advantage of being able to hide the background layer, to see exactly where you have edited. And if your editing isn't quite perfect, it makes it easy to fix it, as shown in Steps 13-16.

I'm working with the tool at its default Strength of 100%.

I've painted over the buildings.

When working around the edge of the cannon, (which we don't want to blur), zoom in and use a smaller brush, for greater accuracy.

Even when you try to paint accurately with a brush, it's difficult to be 100% accurate. It's easy to go over the lines. It's important to blur the buildings completely, up to the edge of the cannon, or it would look odd. In doing this, I've gone slightly over the edge of the cannon.

This is easily checked if you worked on a new empty layer, as I detailed in Step 10. If you edited the original image, or a duplicate layer of it, you won't be able to do this.

If you have edited on a separate layer, turn off visibility for the image layer.

Select the Eraser tool.

For more information see Eraser Tool.

Zoom in to work accurately.

Adjust brush hardness to around 80%. The top of the cannon has a hard edge, so we'll need a brush that's fairly hard.

For straight edges, here's a great technique, which works with all Photoshop's brush tools. Click at the start of the line. Just one click.

Then hold down the Shift key and click carefully at the other end. A brush stroke is automatically created in a straight line between the two points.

The green circle shows where I clicked with the brush in the previous step. On the right the black circle shows where I clicked, while holding down the Shift key. A straight line was brushed between the points.

Provided you used a brush with appropriate hardness and clicked accurately at each end, this will do a perfect job, quickly.

Here's the finished section.