Please help in this

My phone is sony xperia acro s

And I try to install Rom
[ROM][AOSP] - Android 5.1.1 - Beta 2
But I did not understand clearly installation steps
There was a particular topic to explain the installation
[Guide] [XAS] [UPDATED] Xperia Unlocking BL, flashing ROMs/kernels (BPaul's 5.1.1 B2)
I tried to take advantage of it this topic to learn how to install, but it failed me again
I ask you if there is one of you can explain these steps in detail Lee
I tried a lot to it, but did not succeed in one step and come on
unlocking bootloader
Please help me thank you