As said...one wire at a time...cover with shrink wrap...then do the other.
Tin both terminals of the deans connector
Cut red wire...tin the end...place shrink wrap on wire...solder to +ve terminal of deans...clean up any jagged bits with a file. Let cool...position shrink wrap...heat shrink in place.
Cut black wire...tin the end...place shrink wrap on wire...solder to -ve terminal of deans...clean up any jagged bits with a file or sharp exacto (be careful not to nick the +ve terminal!!). Let cool...position and heat shrink.
Use an 80W+ soldering iron...the lower powered ones can take too long and transfer a lot of heat to the battery (not good).
With a bit of practice it takes no time at all. Practice on some spare wire first. Longer bits of shrink wrap are nicer than the little stubby bits that comes with most deans kits....automotive section of Can Tire.
I use plain lead free flux core solder....others prefer higher end stuff.
When tinning the wire end it's sometimes helpful to strip a bit more wire than needed...tin it and then trim it to length with a wire cutter. Get the wire/solder hot enough so that it flows into the wire strands or "floods" onto the deans terminal. If you find yourself "pushing" it on...your soldering iron isn't hot enough.
Best of luck.