I am creating a Half-Dragon subrace referencing this site. Of course, I'm changing things, not just plopping things into the race, but one of the things I want to add is this: Draconic Nature. Your creature type is considered to be both dragon and humanoid. How can I do that?

Can’t you give it both tags? That would at least get it part of the way. To be honest I’m not sure you can actually give it both types/subtypes

Yeah, but how do you do that? is it under the modifiers section? if so, I can't find any way to do it...

You cannot change creature type for player races. However, since this affects absolutely nothing, (character sheet wise), is not a searchable thing on player races in the homebrew list and is not referenced anywhere on the sheet - just having the text alone is sufficient.

In some cases, this is not true.  For example, if you want to make a homebrew elf race, you need to change the creature type to include elf so your race can qualify for feats like Elven Accuracy.