The receipt printer must be connected to your router. By default it uses DHCP to obtain the IP address, if you are using fixed IP's then you will need to configure the receipt printer(s) with a fixed IP address using the software provided with the printer.
Next you'll need to connect the cash drawer to the receipt printer.
Now, download the 'Sellr POS' app from the Apple App Store and install it on your iPad.

Once you have the hardware set up as above, you should now set up a user to use the POS system.

On the Sellr control panel click 'profile'>'user'>'logins' and create a user. Make sure they have access to the 'sales' section.
Now switch back to the iPad, run the app and log in.

You can now connect your receipt printer(s) to the app.

To find out the ip address of a printer, switch it off using the switch on the side. Wait for 10 seconds then switch it on using the switch on the side, also holding down the round switch on the front. Keep this pressed down until the printer prints its network status report after about 20 seconds. This report will contain the IP address of the printer.

On the Sellr POS app, click on the 'settings' button and enter the IP address of your receipt printer(s).

The system is now ready to use.

Make sure you have some POS payment methods set up on Sellr by clicking 'configure'>'payment methods' in the drop down menu at the top.