Hi there,
So it took me a while to get it correct, mainly Do not make the hole too big the pad can be oval but the hole can’t be too big or the solder will flow around the pad and not onto it, Yours LQQK a little big ,if that makes sense, I use flux only initially just to hold stuff in place. Generally I don’t tin anything but will put some solder paste on the BAT PADS very neatly. The holes are a “PLATED through Hole” variety, BTW the plating helps transfer the heat from the bottom to the top with the tip of the soldering iron IN the hole against the side wall and the solder being fed in from the side. I fill the hole up slowly and if it’s flowing down into the hole I stop let it receded , you can tell as soon as you remove the clamp if your good or not. it’s on there!
then I flip it over solder only the pins I need to use on the top. Plug in the battery and plug in the usb if it’s Green I’m Clean. 
HTH
GL 
PJ
