plain sailing

Short definition from Wiktionary

Noun

1. 1
   The navigation of waters free from hazards or unfavourable winds.
2. 2
   Obsolete spelling of plane sailing (“a technique for navigation using the assumption that the journey occurs over a plane or flat surface rather than the actual curved surface of the Earth, which is sufficiently accurate over short distances”).
3. 3
   Something that is easy, simple, or straightforward; something that offers no difficulties or trouble.

Adjective

1. 1
   Easy, simple, straightforward.