Well, what do you need help with?
This is a good starting point if you never have built extensions before https://help.qlik.com/sense/2.0/en-us/developer/Subsystems/Extensions/Content/extensions-introduction.htm
However you will probably be hard pressed to build all of this as an extension.
There is currently no way of hooking into our routing to add your own routes, I guess you could to query parameters and pass in a bookmark id and try to parse that.
However I would probably instead build a "bounce" page that would get the app, the bookmark id and then behind the scenes open that app and apply the bookmark and then forward the user on to the app to the correct sheet. That would select the appropriate values in the users session so the selections will already be applied when they reach the app.
That seems like a lot of work though. We have sharing of bookmarks on the roadmap so if they are willing to wait we will ship that feature, eventually
I would suggest you start by reading the relevant section in the documentation
Stefan Walthers extension tutorial is also a great starting point