Promesa

The Mate & the Jersey can be found on the 10th level using stage select (Thumbnail is inside of an apartment building with a window and staircase) progress through the level and you'll eventually come to a kitchen and the Mate/grey cup with a drinking straw with be on the left side counter next to the oven, simply look at it and the Achievement will unlock.

I show the location in my Achievement Walkthrough starting at 2:41 in the video.

Located on the tenth level if you're going by Scene Selection, otherwise this is the level where you have to go down many flights of stairs. Begin by going all the way down the stairs to the bottom. From here, take a sharp right and go outside. If you look up and to the right you'll see some shirts hanging on a balcony, look at the red one until the La Camiseta achievement pops. From here walk to your left and past the red scooter, taking another left down the alley. Walk through here to get warped to the kitchen area and then walk inside. If you look to your immediate left on the counter you'll see a grey cup with a straw to the right of a microwave, look at it until the Mate achievement pops.

I recommend finishing the level after this instead of quitting out, when I quit out the Scene Selection option vanished and I had to force quit the game to make it reappear. To end the scene just look at the golden bicycle at the far end of the kitchen.

After beating the game you unlock stage select.
On the 10th stage there's a gray cup you need to look at for an achievement

See in video at 6:40
Achievement Walkthrough: