Don't use old, outdated polish if you want your manicure look its best. As it ages, it'll get thicker and more difficult to use, decreasing its staying power. Either throw out the old polish, or add a bit of polish thinner to smooth it out. Don't confuse polish thinner with polish remover!
It's easy to give yourself a manicure. All you need is some soapy water, less than a half-hour of time, and a few tools: nail clippers for cutting, an emery board for shaping, an orange-stick for cleaning and cuticle manipulation, cuticle oil, and a buffer.
When shaping your nails during a self manicure, be very careful about filing down the sides of the nails. If you overfile the sides, you could end up weakening your nails, causing them to split and crack. It's better to prevent that than to have to fix it later.
