Always follow the TICKS safety rules: Tight, In view, Close enough to kiss, Keep chin off chest, Supported back. 

There are some great videos on Sling A Baby Youtube channel.

Below are detailed picture-by-picture instructions.

Front Wrap Cross Carry (FWCC) is ideal to use from birth. You can use a stretchy or a woven wrap for this carry. This version of the fwcc, with the cummerbund outside is sometimes referred to as pocket wrap cross carry.

 The Rucksack back carry (ruck) is suitable from around 12 weeks old. It can be done with a relatively short wrap. A size 4.2m is usually suitable for most people. We advise the presence of an experienced babywearer when attempting back carrying for the first time.
Carrying your baby on your back requires vigilance and practice. Once you feel confident carrying your baby on your back, the rewards are tremendous.

A Mei Tai is a square/rectangular piece of fabric with two waist straps and two shoulder straps. They can be used on the front and on the back, as soon as the baby has good head and upper back control. Some Mei Tais come with buckles, and are therefore referred to as buckle tais.

A ring sling is a piece of fabric fastened and adjusted at the shoulder with a pair of rings. It can be used from birth.