Accepting a payment with the GUI is very easy. Doesn't matter if you are a merchant or an user, you will have two pages available: Receive and Merchants.
This is a POS-like page that merchant can easily use to set the amount of XMR to receive. If the 'Sale tracker' option is enabled, you will see the payments while they arrive in real time in your wallet, along with the number of confirmations.
Share the address composed by letters and numbers to the person you want to receive coins from. You probably prefer to use the more user friendly QR code.
If you want to specify the amount to receive, go to the 'Merchant' page (after you selected in the 'Receive' page the @account that will be used to receive XMR).
Insert the amount to receive, then share with the payer the payment URL or the QR code. If you want to track the payment in real time, tick the 'enable sales tracker' option.
Wait until the payment has arrived and has enough confirmations (The more confirmations, the safer the transaction is. You need at least 10 confirmations before you can spend the funds.).
