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}\$.
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.)