Just tap one, tap incubate, select the orange case and you’re good.
If you have more than just the orange one (blue or purple), only put 2km in the orange one. The orange one has unlimited uses and you get the most bang for your buck.
The blue is limited use(s) and the purple is limited uses but lowers the amount you have to walk. Use your 5km, 7km and 10km eggs in the blue and purple.
TLDR
- 2km eggs (common pokemon) in orange only
- 5km eggs (less common pokemon) in orange, blue, purple
- 7km eggs (alola pokemon) in orange, blue, purple
- 10km eggs (rare pokemon) in orange, blue, purple