First, get all the runes listed in the recipe. You can either trade the runes or go look for them yourself. Low and Mid runes can easily be found from the Countess and the Hellforge Quest. High runes rely on a lot of luck because they can only be found from a monster or chest drop starting from Hell difficulty.
Make sure to double check your runes, for example Lo and Io are often mistaken.
Find the Correct Runeword Base Item
You need a non-magical, gray item. Ethereal and superior items can be used to create a Runeword. You can not use, magic, rare, unique, set, or crafted items.
The item needs the have the exact amount of sockets as listed in the recipe. If the item has more sockets, the Runeword will not work.
The item type must be eligible for the Runeword. F.e. Insight can only be made in Polearms or Staves (missile weapons too in patch 2.4), so it will not work in Spears (which look like Polearms). Note that for Runewords, Scepters, Maces, Hammers, and Clubs are distinct classes, even though the game groups them all under the Mace class. See the detailed type definition for more info.
When you find a good base without sockets you can try to add sockets with the Horadric Cube socket recipe. If the maximum amount of sockets on the item equals the amount you need, you can also take it to Larzuk, he will add the maximum amount of sockets to the item as a quest reward.
Insert the Runes In the Correct Order As Listed In The Runeword Recipe
Make sure to add the runes in the correct order as the recipe says. If the order of the runes is wrong, the runeword will not work and your runes will be lost.