I have a Hakko FX888D arriving today. Any tips on temp and technique for soldering the center pin on an Eidolic 2.5mm?
Use rosin core solder, heat the joint then apply solder to the joint as you hold your iron tip in place, if the center flux (inside the solder) is not enough to clean the joint you can use liquid rosin flux but rarely should you have to do that. I generally use 800 degrees but 700 should work nicely for your application depending on the tip used. I have used some Hakko irons and I believe the stock tip should be fine for your application. I use a panavise to hold connectors but helping hands devices can work in a pinch and cost a lot less. The joint when completed should be nice and shiny, make sure the solder flows smoothly over the joint.
The connector appears to have a hollow center hole so if you fill that with solder, tin the wire to be attached, then heat the joint and insert the wire you should be fine. The ground has a hole you should try to feed the second wire through once stripped. I would not pre-tin this wire since you want to be able to feed it through, bend it, then solder it for a good solder joint.
When you remove the solder tip from the joint, make sure the joint is not moved while the solder is still in a molten state. Just hold the wire there for 15 seconds or so without the iron tip present and you should be fine. You can always go back and resolder it a bit if necessary.
I just checked the tip size on a new in the box 888d I have here and it comes with a nice size medium tip that should work well with this application. Keep the iron tip clean by running it across a wet sponge (sponge provided with iron) and you should be fine.
If you have any old cables you do not need, you might want to cut those apart and practice soldering a bit, just tinning the end wires, which is good practice when installing wires in a connector solder cup, might be good experience before finally soldering this connector.
Tom