Your first layer will be the base layer. Base layers perform the essential function of wicking moisture away from your body. When you ski, you sweat. Moisture left to sit next to your skin will lower your core temperature, causing you to get cold and risk hypothermia. Base layers, therefore, use materials like polyester blends and merino wool to pull the moisture away from your body and disperse it into the air, keeping you dry and warm. You have two basic options when it comes to lower-body base layers: long johns and underwear. Long johns come in a number of different weights or thicknesses so that you can easily plan around the weather. For the coldest temperatures, close to or below zero, opt for heavier long johns. For mid-range temperatures, adjust your base layer to your own personal temperature preferences. You can skip long johns in warmer temperatures and opt for underwear. Underwear should be made of good wicking materials such as polypropylene, merino wool and microfleece. Always avoid cotton base layers.
The next layer for your legs is insulation. This layer provides the majority of your warmth, insulating from cold outside temperatures and keeping your legs warm and comfortable. Before buying separate insulation, consider the insulation provided in your ski pants. There may be enough interior insulation within the ski pants to negate the need for a separate layer. Like the base layer, you'll want insulation composed of materials that handle moisture well, so avoid cotton. Fleece pants are a good option for insulation. Purchase different weights to tailor your clothing to different conditions. In warmer spring temperatures, you can likely skip insulation altogether and just wear a base layer and ski pants.
Like the other components of your lower-body skiing ensemble, socks wick moisture and provide warmth. It's advisable to wear one pair of socks as opposed to two or three separate pairs. Instead of layering your socks, purchase different-weight socks to provide more or less warmth to meet the conditions of the day. Like your other layers, avoid cotton socks and opt for wool, fleece or blended socks. Socks made specifically for skiing can add a bit of cushioning to high-impact areas and make your feet more comfortable during a demanding day.