To create flexible one-time donations, you can create a merchandise product that is $1, and set to be ordered more than once.
This will let the user type the amount they want to donate into the box beside the product name.
If you'd like to set up flexible recurring donations, you can create a $1 subscription product (Go to Settings → Products → Create New Product → click Subscriptions [Recurring / Donations / Membership / SaaS]) with the same "order more than once" setting. This product type can also be adjusted in many other ways to suit the type of donation you want to make available.