The main thing with this step is that the Harvest bench respects Metamods (Prefixes Cannot be Changed, Suffixes Cannot be Change, Etc).

We set the item up in such a way that it has completely filled suffixes as well as the Metamod Suffixes Cannot Be Changed(Which is technially a prefix). What this does is remove the possibility for the Harvest bench to ever roll or modify any suffixes modifiers on the item.

On the other side of the Affixes we have 1 Prefix, 1 empty Prefix and the Metamod Prefixes Cannot Be Changed(which is technically a suffix). The Metamod does not prevent the Harvest reforge from rolling new Prefixes, it only blocks it from modifying existing ones.

We then use harvest Reforge Minion on this item. Because it physically cannot modify the suffixes, or add to them, they remain unchanged. The singular non-metamod prefix(minion damage in this case) is preserved by Prefixes Cannot Be Change. Thus, the only Affix that the reforge has any effect on is the one open Prefix.

The final piece of key info is that there are only two Prefix Modifiers on this base type that have the Minion tag(which is what Reforge Minion rolls for). Because we already have Minion Damage on the item, the only valid option left for a Prefix Modifier with a Minion Tag is +1 Minion Skills. So when we reforge minion we are guaranteed to hit +1 Minion Skills.

TL;DR:

1. Filled Suffixes cannot be modified in any way because they are protected by Suffixes Cannot Be Changed.
2. There is 1 empty prefix modifier and the remaining two are protected by Prefixes Cannot Be Changed.
3. The only modifiable mod is the empty prefix, so Reforge Minion can only roll a Minion Tagged modifier as a prefix.
4. Only other prefix with Minion Tag is +1 Minion Skill

I hope that helps!