Choose Tools > Style (from the Tools menu at the top of your screen).
Press the S key on your keyboard.
In the Tool Options pane, click Add Style and choose Stroke. You can add as many of the same styles as you like.
Customize the stroke fill, color, width, and opacity:
Stroke Fill: Click the Fill pop-up menu below the Stroke Style pop-up menu and choose a Color stroke for a solid color outline, Gradient to fill the outline with a transition between multiple colors, or pattern fill to fill the stroke with your custom pattern.
Stroke Width: In the number field, enter a value from 1 px to 200 px, use steppers, or Up and Down arrow keys to change the thickness of the stroke.
Stroke Color: Click the color well to choose a color or click to pick a color from the image.
Customize the stroke style and settings:
Stroke Style: Click the Stroke Options pop-up menu, then click Stroke Styles to select any of the stroke style presets. If you've created a custom style you'd like to save a preset, you can do so by clicking the Add button in the Stroke Styles menu.
Stroke Position: Click the Align pop-up menu below Stroke Styles to choose the position of the outline. Inside makes the outline expand into your selected layer, Center makes it expand both inwards and outwards from the edges of the layer, and Outside makes the outline appear entirely outside your selected layer.
Cap Style: Click the Caps pop-up menu and choose how the endings of a stroke or dashes in dashed lines should look like. You can choose from Butt, Round, or Square styles.
Corner Style: Click the Corner pop-up menu and choose how the corners of a stroke should look like. You can choose from Square, Round, or Bevel styles.
Endpoints: Choose a particular shape to use for the endings of a stroke. You can choose endpoints such as arrows, diamond, square, and others for the end and start points of a stroke. Use the percentage fields to scale endpoints separately or together.
Dashed Line: Select Dashed Line to convert solid stroke lines into a dashed ones. Change numbers in the Gap and Dash fields to customize the line spacing.
If you have applied multiple Stroke styles to a single layer and you’d like to rearrange them, simply drag the style up or down the list of styles.
To reset all styles applied to an image, click Reset at the bottom of the Tool Options pane.