Mistress starts to spread your asshole with her anal speculum.
Surely I need not tell you to use lube from all that shall happen from here on in . . .
If you have an anal speculum, insert it and expand it to the point of slight discomfort. Be careful, we don't want any injuries here.
If you have an anal probe, then insert it to the first and smallest expansion point.
If you have only an assortment of dildoes or butt plugs, insert the smallest one.
If you have none of these implements, what the fuck are you still doing here?!?!? Oh, well, insert one nicely lubed finger instead, but make certain that all of your fingernails are properly, smoothly trimmed, and clean and that you have no hangnails.