How to send supplies to allies?

To send supplies to Allies you need to be a Trader. Only they have this perk.

Find the player you want to help either by locating their base in the world (they could send you their location). Or by finding them in the alliance member list. (Alliance menu -> tap on members at the top)

Tap on the base or member in the list and there should be a trade option. This will take you to the trade interface where you can decide what to send.

The amount you can send will depend on the level of your Trader exclusive Market Building. Upgrade it to let you send more resources.