I assume you've already familiarized yourself with the `MonteCarlo.asc` file in the educational subdirectory of LTspice. So I won't belabor those details.

What I'll do is grab up a schematic I have -- it's a circuit that once triggered by a manual switch will continue to power a circuit for a specified period of time. *(\$R_{_\text{DEVICE}}\$ is the circuit being powered.)*

It uses a MOSFET device as part of the circuit. So I've modified my schematic to perform a Monte Carlo on \$V_{_\text{TO}}\$ of the **BSS123**. Note that I constructed my own `MODEL` statement and included the **VTO** parameter there, using a 5% tolerance around the typical value of \$1.6\:\text{V}\$.

[![enter image description here][1]][1]

You can see just how much variation in the circuit timing would be due to even that small amount of variation.

Hopefully, this shows you how. If you are using `.SUBCKT`/`.ENDS` models, the same thing applies. But you can also include parameters into these that can then be used with specific parts within the sub-circuit. It all just works fine.

*(I didn't use resistor or capacitor tolerances here. This is just to demonstrate how to modify active device parameters. Not to exhibit this circuit, in particular. It's just an example to make a point.)*

The device parameters for the **BSS123** exist in the standard library file. But they may just as well have come from ***any*** library file you might include. You will need to write your own model statement, of course. But the `ako:` allows you to pull in all the library parameters that you don't want to vary. So it's really easy to vary only the parameters you want to modify.



  [1]: https://i.sstatic.net/mZjojPDs.png