My understanding from what others have posted in the forum is that you shouldn’t need to use custom code at all. Just use the official Smart Life app integration.
I know @Terri_Baker had these working this way. (“C2c” in the following post just means “cloud to cloud“ through the official integration that you will find in the smartthings app when you select SmartLife.)
Some people have said initial Alexa set up can be a little tricky because it creates multiple devices, one for the fan and one for the lights, and they found they had to rename the fan in order to be able to get control of the fan speeds by voice, but it’s become quite a popular device in the community. the last year or so. 
As far as adding devices which don’t have an existing smartthings integration, that’s a whole other conversation. For Wi-Fi devices, the manufacturer usually needs to have an open API. But if you’re interested, you can find out more about that in the following section of the forum. ( note that the groovy cloud is scheduled to go away by the end of this year, so most people aren’t writing new integrations with that anymore. Instead, they are using the new platform. So while there’s a lot of stuff in the forum about the groovy methodology, since you’re starting from scratch I think it might make more sense to go with the new architecture.)