A couple factors to answer your question i'll try to make it as simple as possible.

1. having a good business plan.
2. check your countries regulations.
3. Talk to your bank.
4. secure a good location with high foot traffic.
5. Choose the BTM type and manufacturer: There are one-way and two-way BTMs. The latter allows customers to sell bitcoin for cash as well as buy bitcoin, but they are more expensive.
6. Plan for customer support, and maintenance.
7. If you're looking to run it solo. or with a partner also bring in other factors, like payment sharing, it would make it easier for you to also keep it running due to responsibility sharing, just make sure you think these things through carefully.