There are 15 maps total that you need to get Platinum/Gold/Silver/Bronze level on Time Trials. They are selected from the level select screen and they seem very daunting when you first approach them. After you start to get more practice things start to come together much smoother and the main key is determination. A couple tricks I learned from the attached video guide is to use the boost jump (Tapping
while airborne) regularly, especially going uphill and gravitate
on any small downslope to keep your speed high.
The following guide by Rubhen925 shows great pathways for each level and he doesn't do anything that is insanely difficult. I found a couple things to note but otherwise you can just watch and practice his routes until you make the time you need. Here are two things I noticed I think others might appreciate.
On the level Into the Woods where you are first taught to gravitate upside down I would consistently smash into an invisible wall where there is a noticeable trough in the metal pipe. I couldn't figure out how to avoid it although on two separate runs it didn't happen. It will make you want to pull your hair out but it is possible to Platinum even with the abrupt stop.
In the final trigger trial Archipelago in the video he veers hard left to collect time and boosts. I found it just as easy to complete this trial without that maneuver.
Good Luck everyone!