Don’t use a blunt razor

Shaving with a blunt razor blade increases the likelihood of razor burn and skin irritation. If you start to feel your blade pull at your body hair or feel rough on your skin, replace it.
Hydrate your skin before you shave

It may be tempting to dry shave your underarms given how small the area is. But keep in mind the skin under your arms is sensitive, so it’s important to protect it while shaving.
Soak your underarms in warm water in a shower or bath for around three minutes to soften your skin and make the hair thick with water. This helps the hair cut more easily.
Wash your underarms

Taking a bath or shower before you shave can ensure your underarms are clear of sweat and deodorant which can clog razor blades.
Washing helps the razor more effectively shave off body hair; it also helps prevent bacteria from the combination of sweat and deodorant from penetrating through the skin if scratched while shaving.
Use shaving gel

Although the surface area is tiny and only takes a few strokes of a razor to shave, remember that underarms are sensitive. Use shaving gel to maintain moisture and help the razor glide easily over your skin, to better protect from irritation.
Shave your underarms in all directions

The hair under our arms grows in all directions, so shaving in one direction might not result in a close shave. For best results, lift your arm above your head, press gently down on the razor and shave lightly in short strokes in all directions — downward, sideways and back upward, if needed.
Be sure to moisturise

Once you’re finished shaving, rinse and gently pat your underarms dry with a clean towel. Apply moisturiser or oil, preferably one that’s alcohol-free. Although it may sound counter-intuitive to moisturise this area, it will help keep your skin hydrated and reduce the chances of developing razor burns and bumps.
Wait before applying deodorant

After you shave, wait a little bit before putting on deodorant. Applying deodorant to freshly shaven skin may cause some discomfort. So, allow your skin time to rest to reduce the chances of irritation from the deodorant.