https://www.youtube.com/watch?v=0FNQTxX_jT4
https://www.youtube.com/watch?v=v4Yi5Cy2VMA (also several other good tree ones and other terrain ones by Mel)
two of my fave channels and tutorials
But also, I have bought and love JTT brand scale trees- however, unless you get them on sale, they can be quite expensive certainly compared to making your own. Totally worth watching for however!
As for basing, I'd really say either craft plywood (basswood) or
MDF is best. You can use hardboard but
imo it's a bit thin generally for that purpose alone, maybe if you make it doubled near the tree itself. Anyway just some thoughts!