3x9 speed to a 1x9 speed conversion I want to convert my 27 speed 3x9 to a 1x9. What do you recommend for chainring and cassette combinations? What are the cautions?
To figure out what cassette and ring size to get, the best approach is to use a gearing calculator to play with the options in comparison to what you need or would like based on your experience with bike's current gearing and how it's working for you where you ride. With 1x conversions the usual question is how much high end you can accept losing to get the low end you need or want.

The cheap way is to figure out what ring to go with based on the above and then get a narrow-wide ring of the right BCD to achieve it. Do all chainline measurements up front and use chainring spacers, a different BB spindle, and/or an offset chainring as applicable to achieve perfect chainline.