a) first of all - this ROM is not similar to stock ROM. Many features miss like some Sony stock applications like Messaging or Video. There are also not XPERIA Themes or Stamina mode included. But ROM is fully working and it is very similar to clean Android. You can any time switch back to stock Sony ROM. But try it, you will like it.
b) download and install latest Flashtool from this article
c) download also latest Sonyś Android Concept (mirrors Google Drive or Mega) for Sony XPERIA Z3
d) paste the ftf file downloaded in step c) to folder „Firmware“ included in the XPERIA Flashtool
e) open the XPERIA Flashtool, click on flash and choose downloaded firmware, than click ok. Important - if you do not want to delete any data of your phone, just uncheck flashing userdata.sin file. All data will remain in your phone, but sometimes it can cause additional troubles. So it is better (and I recommend that) to do the backup of your personal data, than proceed with clean instalation and then restore your personal data.
f) turn off your phone and when Flashtool shows pop up window, connect phone to PC with holding volume down button. When Flashtool ask you if use FSC script, hit NO
g) after few minutes all is done and your phone gets the new Android