It’s a good idea to look over your skin about once a month. This allows you to get to know your skin and what your skin looks at baseline. Anything new or changing on top of your first exam would be of concern. Secondly, anything that looks suspicious, especially via the ABCDE criteria for melanoma, would be of concern as well.
Having a full-length mirror and handheld mirror can be helpful tools. First, look over the front of your body: your face, neck, chest, arms, belly, thighs and legs — including under the undergarment areas. Examining the back side of your body can be a little more difficult. Using a hand held mirror along with the full-length mirror can allow you to visualize the back of the neck, the back, buttocks, and back of the thighs and legs. Also look at the hands including the palms, and soles. Check behind the ears and through the scalp as well. If you don’t have a hand held mirror or find that you still don’t have good visualization of certain areas of your body, you might to have someone examine your skin for you (e.g. a partner or spouse). Your barber/hairdresser can also help you look through your scalp. If any of the spots on your skin are concerning to you, you should let your dermatologist know even before your regularly scheduled skin examination.
The American Academy of Dermatology (AAD) has a good webpage diagramming the steps to the self-skin exam.