[VStar650CL]

On late models like your '18 the armrest cushion is part of the panel assembly and not sold separately. They're usually plastic-riveted to the panel, so if you're replacing it using a used or junkyard part, it will probably need to be drilled loose and the replacement glued in.

 

[VStar650CL]

No, just use a nasty adhesive like Permatex Ultra Gray and make sure you use alcohol on the mating surfaces. If you can get the replacement cover from aftermarket, installing it shouldn't be a big deal. I don't have a Sentra door panel here to look at, but maybe you'll even get lucky and find it screwed on instead of riveted.