What you are trying to do is actually quite simple. All you need to do is weld the part to the Torso. While this may be quite simple, I can see how it can be complicated when you've had little experience with welding, so I will demonstrate how welding works below and if you want to know more about welding, click here.
Welds are objects that you can use to join parts together, very useful for weapon creation, custom character joints, and such. The 4 main properties of Welds are: Part0, Part1, C0, and C1.
What the part is being connected to.
In a Character it would be the Torso.
The part itself.
The part that is being connected.
If the weld is for the RIght Arm in a Character, Right Arm would be the Part1.
C0 and C1
Both CFrame values
They both decide the offset for the parts.
I don't want to go into details of what each is because anyhow I explain C0 and C1, the wiki can describe them better.
Anyways, that's my explanation of what a Weld is and it's uses.