There are several ways to make a mask. Here are three methods that I commonly use for creating custom masks:
Manual cut–Using your lasso tool, loosely draw a shape around the area you want to mask. Once you have a selection, click the “save selection as channel” button (located at the bottom of your channels window). Then, click the “eye” on the mask channel and the “eye” on your CMYK channels. This will give you a live preview of your mask as you edit it. Using your lasso tool, continue to cut the mask adding or removing black until the mask is complete.
Masking from layers–Starting with your top layer, select the layer, then click the “save selection as channel” button (located at the bottom of your channels window). Repeat the process until all your layers have been converted to channel masks. Next, go to your channels and start from the last channel you made (on the bottom). Select the channel and create a selection (command-click) on the channel above it. Then, add to the selection (shift-command-click) on all the masks above it. Once you have the selection, you will fill it to black on your mask. This knocks out any layers that were on top. Repeat this process for the remainder of the channels. Once you have completed all the knockouts, combine the masks that belong together.
Masking from color range–Using color range, you can create masks for the standard colors, or you can use the eyedropper to select colors that are not on the default list. When choosing a custom color, I recommend starting the fuzziness at 50, then adjusting as necessary. Choose a color with color range to create a custom selection, then click the “save selection as channel” button. Repeat this process for each mask you wish to make.