If the sender is determining the amount being sent, all they have to do is click on "Send", enter the receiving address (your address under the text: "Your Current Address") or scan your public address, (QR code displayed on the main wallet page) and specify the amount.
If you are the one who specifies the amount, you can generate a Payment Request QR code to request a specific amount of funds. To do so:
1. Select Receive at the top left of the main Jaxx display.
2. Select whether you want the amount to be in the currency type or the default fiat currency of your choice.
3. Enter the requested amount into the text field.
4. Select Generate.