Ghouls and goblins are terrorizing the streets, but sometimes deciding on your costume—or kosuchūmu—can be even scarier. The structure “X ni shimasu” is used when expressing a decision you’ve made after mulling over the options. Placed directly after the noun or gerund of your choice, it translates to “I’m going with…” or “I’ll make it…”
How Would You Like That?
JP: Kosuchūmu wa nani ni shimasu ka?
EN: What will your costume be?
JP: Sō desu ne. SF-kei ni shimasu.
EN: Hmm… I’ll make it sci-fi-themed.
EN: For example?
JP: Uchū-jin no kosuchūmu ni shiyō to omotte imasu.
EN: I’m thinking of going with an alien costume.
JP: Sō desu ka. Dewa, watashi wa uchū-hikōshi no kosuchūmu ni shimasu.
EN: I see. Well then, I’ll go as an astronaut.