To make diagonal stripes in Photoshop, first make a new layer. Then, using the Pen tool, draw a line from one corner of the layer to the opposite corner. To make the line thicker, use the Stroke option in the options bar. Choose a color for the stroke, and set the width to something like 10 pixels.
Next, go to Edit > Transform > Warp, and choose the Arc Upper preset. Drag the vertices of the line until you have a nice arc. Finally, go to Edit > Transform > Distort, and drag the top and bottom vertices of the line inwards until you have a wavy effect.
PRO TIP: If you are not careful, making diagonal stripes in Photoshop can result in some very jagged and uneven lines. Be sure to use the correct brush size and hardness, and take your time with each stroke to avoid any unwanted mistakes.