How can I download Cydia on iOS 7?

You don't download Cydia. An exploit is used to bypass certain restrictions and the Cydia application is injected into the device through the jailbreak software. 
Choose the jailbreak tool corresponding to your firmware (what version iOS) below:
7.0.0-7.0.6 Evad3rs Jailbreak located here: http://evasi0n.com

7.1-7.1.2 Pangu Jailbreak located here: http://en.7.pangu.io

Google for a tutorial if you need more help,Must Read:erase iPhone without password
for example: if you have a device on 7.1.2, you would Google something like:
"How to jailbreak iOS 7.1.2 using Pangu"  Must Read:how to import contacts from iPhone to mac?

Hope this helps. Just be glad that there's an English version of Pangu now XD Jihosoft iPhone Backup Extractor Extract and recover all types of data from iPhone backup files, such as camera roll photos, videos, SMS, contacts, reminders, WhatsApp, and etc.

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