Tap the Neighbours button in the lower left corner of the screen to see the list of your neighbours.
Next, tap a neighbour's picture. If it has a purple lightning icon in the corner, it means that you can help this neighbour in their city.
Once you're in their city, tap any building that has a coin or a timer icon above it to help with this building. You can help a neighbour up to five times per day. Lightning icons in the top right corner show how many times you can help this neighbour today.
When a neighbour helps you, you will see their picture over one of your buildings. Tap it to accept their help.