If you don’t have the original paperwork or know exactly which thread pattern you have, you will want to measure it before ordering a suppressor online.
There are two parts to determining the thread pattern for your firearm:
PITCH - measure this with a thread checker. We bought ours at a local hardware store for less than $10
LENGTH - measure this with calipers or a ruler.
The barrel in the photo here is a 5/8 x 24 thread pattern.
See video below for more details.