The first step is to create a new plugin using our PluginSkeleton.
$ composer create-project sylius/plugin-skeleton IronManSyliusProductOnDemandPluginNote
Remember about naming convention! Sylius plugin should start with your vendor name, followed by Sylius prefix and with Plugin suffix at the end. Let's say your vendor name is IronMan. Come on IronMan, let's create your plugin!