To brush on a selection, choose the Select and Mask option from Photoshop’s Select menu. The Select and Mask window then opens. On the toolbar on the left side of the screen, choose the third option from the top. This is the Brush Tool. In the options bar above it, you’ll see the plus sign selected by default; this adds to the selection wherever you paint. To remove from the selection, click the minus sign or hold the CTRL key while you click and drag over the area to be deselected.
Next, and perhaps most important, is the brush size and hardness. I like to use as large a brush as I comfortably can, based on the size of the area to be selected. And I typically prefer to use this technique for soft-edged selections, so I dial down the hardness. For a well-defined selection edge, use a harder brush.
You can change how the selection is displayed by changing the View Mode in the top of the Properties panel on the right side of the screen; I prefer the red overlay with selected areas transparent.
Lastly, you’ll want to be deliberate with what the painted area is output to—a selection or a mask. For my retouching purposes, I prefer to paint the area to a selection, but depending on your own needs you may find it useful—and a bit of a time-saver—to output the painted area directly to a layer mask. This is adjusted at the bottom of the Select and Mask window.
When you’re done brushing on the selection, click Okay at the bottom right of the screen, and watch as Photoshop shows you the results of your efforts, in a “marching ants” style selection or a layer mask depending on your choice in the previous step. Once you get used to the process, brushing on selections is an incredibly efficient selection process.