Open you system/iteminfo.lua (if you don't have one, just download him here in rathena).
Add in last line of the file (to make more easy to found, or you can add following the item id order)
[<item id>] = {
unidentifiedDisplayName = <item name to show when not magnified>,
unidentifiedResourceName = <file name prefix used for all the images and drop sprite when not magnified>,
unidentifiedDescriptionName = { <comma separated list of strings>,<to get multiple lines>,<in item description> },
identifiedDisplayName = <item name to show when magnified>,
identifiedResourceName = <file name prefix used for all the images and drop sprite when magnified>,
identifiedDescriptionName = { <same format as unidentifiedDescriptionName but for magnified items> },
slotCount = <number of slots>,
ClassNum = <View ID - yes the same one that was there item_db>
},
Example:
[20302] = { unidentifiedDisplayName = "Hat", unidentifiedResourceName = "ĸ", unidentifiedDescriptionName = { "Unknown Item, can be identified by ^6A5ACDMagnifier^000000." }, identifiedDisplayName = "Costume Beelzebub Crown", identifiedResourceName = "Cº£¸£Á¦ºêºêÀÇ¿Õ°ü", identifiedDescriptionName = { "A crown of MVP Beelzebub from nameless island. Its give noble atmosphere.", "Class: ^777777Costume^000000", "Defense: ^7777770^000000", "Location: ^777777Upper^000000", "Weight: ^7777770^000000", "Required Level: ^7777771^000000", "Jobs: ^777777All^000000" }, slotCount = 0, ClassNum = 1343 },