Two clarifications are needed in section '10.5. Poudriere' of the Porters Handbook.
1. Poudriere does not creates a directory DISTFILES_CACHE if does not exist, see https://github.com/freebsd/poudriere/issues/285
I suggest to add at the end of DISTFILES_CACHE description (in "10.5.2. Setting Up Poudriere") something like the following.
You might need to create the directory manually if poudriere complains that it does not exist.
2. There are changes in how to use manually managed ports, see https://github.com/freebsd/poudriere/wiki/use_system_ports_tree . I think, this is to be reflected in '10.5.6. Using Manually Managed Ports Trees with Poudriere'.
(I did not use the new method myself. I made a symlink before I found the description, but I cannot recommend doing that...)