You’d imagine this would be pretty straight forward right? Something like, VS2015 and then install SSDT right? WRONG!
First you need to install VS 2015 and then install the LATEST version of SSDT available here. After this the install should go through OK.
The issue is when you do a <favourite search provider> search for ‘SQL Server Data Tools’ you come to this page and you are offered two options, pick the top one: