In each level you can earn Gold, Silver, and Bronze ranks by collecting 10, 8, and 6 golden stars, respectively. To earn these two achievements, you need to earn Gold ranks on each level of two complete level packs. Insane Hills and Stadium Stunts are the easiest so it is suggested you attempt to get all of the stars on these two level packs.
Stars are collected when your snowboarder gets close enough to them (or runs into them). To earn a Gold rank, you need to collect all 10 on a SINGLE playthrough of the level. There is a trick that makes this easier. As soon as you see a star you missed, immediately fall off your board or hit the restart button in the top left corner of the screen. You can do this as many times as you want. This will prevent you from truly having to collect the stars in one go of the level, because you can play each section between checkpoints indefinitely.
Often you will need to explore a little bit to find all 10 of the stars. In some rare cases, there are actually more than 10 stars on a level, so you can miss some.
*GLITCH WARNING* This achievement is known to be glitchy. Sometimes it will unlock before actually having Gold ranks on all levels required. Other times it will not unlock when you have gotten all the stars in two level packs. If your case is the latter, load-up Time Trial Mode and complete a few levels in Insane Hills, collecting as many stars as you can. This should pop that achievement for everyone, but it is not guaranteed.