I guess you are talking about CPFP (Child Pays for Parent) CPFP is a transaction that somewhat "speeds up" your transaction even though it has been sent and the fee wasn't enough for it to get confirmed. What it is is that basically, you send some amount with a lot higher fee to your address which generates an unconfirmed TX, but will be confirmed because of the higher fee, therefore confirming the other TX.
To do a Child-Pays-For-Parent transaction, you need to find out the "ideal" fee. 21.Co is a good place to look at (
http://bitcoinfees.21.co/) and after that; You can use mocacinno's fee checker and CPFP tool here;
https://www.mocacinno.com/page/feechecker which explains everything in detail.
Thank you for your useful informations. I also need to know, where to send the second payment. Do I send this to my own wallet address or once more to the adress where I sent it before? You wrote "you send some amount with a lot higher fee to your adress", so it is to my own adress, write?