The Blood Knot makes a thinner, neater connection than a Surgeon's Knot — important if your leader is going to ride through the rod tip during casts (a Surgeon's bumps audibly through guides; a Blood Knot glides).
Wrap each tag end 5 times around the standing line of the other piece, then thread the tags through the central X formed where the lines cross. Wet, pull all four ends slowly until it seats.
The downside: harder to tie cold-fingered in the wind. Most anglers use Blood Knots for at-home leader construction, Surgeon's Knots for stream-side tippet refresh.