You need to have a basic understanding of coding in order to do xpath. The thread itself is not badly written, you just have to be at a specific level first. I started creating my own mod about a few months ago and the link I provided was an excellent source of information to get me started.
Editing the games xml files themselves is not the best way of doing mods as they get updated by steam all the time (and at hotfixes).
This is an example of something I have written up to modify the items variables after the game loads up the original items.xml file and my items.xml file
<config>
<!-- *************************************************************************************************************** -->
<!-- This section covers changes to weapons -->
<!-- Changes to Rifles, including T0 changes -->
<append xpath="/items/item[@name='gunRifleT1HuntingRifle']/property[@name='Tags']/@value">,T1Rifle,T1Crafting</append>
<append xpath="/items/item[@name='gunRifleT2LeverActionRifle']/property[@name='Tags']/@value">,T2Rifle,T2Crafting,revolver</append>
<append xpath="/items/item[@name='gunRifleT3SniperRifle']/property[@name='Tags']/@value">,T3Rifle,T3Crafting</append>
<append xpath="/items/item[@name='gunRifleT0PipeRifle']/property[@name='Tags']/@value">,PipeRifle,noMods</append>
<set xpath="/items/item[@name='gunRifleT0PipeRifle']/effect_group/passive_effect[contains(@name,'ModSlots')]/@tier">0</set>
<set xpath="/items/item[@name='gunRifleT0PipeRifle']/effect_group/passive_effect[contains(@name,'ModSlots')]/@value">0</set>
<remove xpath="/items/item[@name='gunRifleT0PipeRifle']/property[@name='RepairTools']"/>
<set xpath="/items/item[@name='gunRifleT0PipeRifle']/property[@class='Action0']/property[@name='Magazine_items']/@value">ammo762mmBulletBall</set>
<set xpath="/items/item[@name='gunRifleT2LeverActionRifle']/effect_group/passive_effect[contains(@name,'EntityDamage') and contains(@operation,'base_add')]/@value">-11</set>
<set xpath="/items/item[@name='gunRifleT2LeverActionRifle']/property[@class='Action0']/property[@name='Magazine_items']/@value">ammo44MagnumBulletBall,ammo44MagnumBulletHP,ammo44MagnumBulletAP</set>
<setattribute xpath="/items/item[@name='gunRifleT0PipeRifle']/property[@name='DegradationBreaksAfter']" name="value">true</setattribute>
This snippet of code (along with code changes to the recipes and progression files) allowed me to limit aspects of the pipe rifles, change the ammo type of the lever action rifle, and introduce Q6 crafting ability through a gated approach. Guppycur also has a discord channel where those who mod or start to mod can go for assistance and ask questions.