okay, to start with, i'm going to assume you have kotor tool, as well as the .gff editor (both can be downloaded from the main kotor downloads sticky at the top of the page).
i'm also going to go through it rather quickly, as you can pick it apart step by step with kotor tool open.
okay, first decide what item you want to modify (in this case, i just chose the regular blaster rifle) and find out its item code (you can find a list of every item's code at various places on the net, i think i used
www.gamefaqs.com).
now fire up kotor tool, and open the tree labeled "BIFs", and then open "templates", then "Blueprints, Items".
you'll find g_w_blstrrfl001.uti, double-click it.
on the first tab, you'll only be changing one thing (everything else must be changed with the .gff editor, i don't know why): chage the tag to my_w_rifle, or whatever you want----BUT you must be consistent with this name.
next click on properties tab, and double-click the "*". here's where you add all the cool stats to the item. i don't remember all i did to your rifle, but you can do pretty much anything here, and they're mostly self-explanitory.
for fun, let's give it a +4 to damage. from property name, select "damage bonus". for subtype, select "energy". and for value, select "4".
now, if you want to make the item upgradeable, you must select properties like you normally would, but also select an upgradetype.
for fun, let's give this an attack bonus when you put in a hair trigger. select "attack bonus", and "3". then go over to "upgradetype", and select "hair trigger". now when you put a hair trigger in this item at the upgrade screen, it will add a +3 attack bonus.
finish adding all your properties, and save it--- REMEMBER the tag you changed at the begining, use the same name.
now, we're almost done, open up the .gff editor. find where it says DescIdentified. Look to the right, and change the STRREF to -1, and the language id to 0. then in the big white box, type the description of your item "this rifle is blah blah blah..."
next go to LocalizedName, and do the same, but here is where you put the name of your item. "Kellen's Rifle", or whatever.
finally, make sure Tag and TemplateResRef are the same (they won't be, you'll have to change one of them). and now save it as my_w_rifle.uti (or whatever you named it).
i know i made this sound long and complicated, but if you do this step by step, it takes about 5mins, and it is so simple. lightsabers are a little more difficult, but not much.
hope this helps.