I'm done with most of the preparations but there are still some little things left.
I'm almost done getting ready but there are still some little things left.
こんにちは。
様々な言い方ができると思いますが、例えば以下のような英語表現はいかがでしょうか：
I'm done with most of the preparations but there are still some little things left.
ほどんどの準備は終わったけどまだ細かなことが残っています。
I'm almost done getting ready but there are still some little things left.
ほとんど準備は終わったけどまだ細かなことが残っています。
little things で「細かなこと」のようなニュアンスになります。
ぜひ参考にしてください。
Preparations are mostly done, but there are still some details to be worked out.
"準備はだいたい終わったけど、細かなことが残っている"
- Preparations are mostly done, but there are still some details to be worked out.
- The preparations are mostly complete, there a just a few details left that need to be sorted out.
- Everything is almost complete, there are just a few things left.
アメリカ合衆国 
