After keying in an amount and leaving the General Fund as the account, you can do a single giving transaction or set up recurring giving. The image below is for a single transaction. (Recurring giving probably requires using the Sign Up link to register with Tithe.ly)
When the “Give $…” button turns from gray to green, you’re ready to go. Click and you’re done.
There is also a Tithe.ly phone app. You can learn more about the Tithe.ly service at their Frequently Asked Questions page.