In Windows, open Disk Management and shrink one of your drives to create a new unallocated partition (at least 16GB formatted to NTFS or left unallocated) where Phoenix OS will live.
Based on the query, it appears that users are looking for a reliable source to download Phoenix OS TL Mod V40. The addition of "best" in the query implies that users are seeking a high-quality or recommended version of this mod.