For the following steps you will need a grid every 1px. So, go to Edit > Preferences > Guides & Grid and enter 1 in the Gridline every box. Pick the Rectangle Tool(M), create a 28 by 42px shape, fill it with R=255 G=221 B=23 then go to Object > Path > Add Anchor Points twice. Switch to the Direct Selection Tool (A) and focus on the top side of this yellow rectangle. Select the left and the right anchor points and simply drag them 2px down. Reselect this yellow shape and go to Effect > Warp > Arc Lower. Enter the data shown below, click OK and go to Object > Expand Appearance.