Pre-tin the component leads ! This will save you a great deal of trouble and time. Put a ‘healthy coat’ of lead-free solder on each lead. Really, put as much on as the lead will take and still fit through the eyelet or through-hole on your DPCB.
When ‘tinning’, use the same solder from the same roll as you are going to use to solder to the DCPB. If you have not tried this, you may think this is a trivial suggestion. After you try it, you will become an advocate.
The reason is that when you are building with parts from your “junque box” or the parts laying around your laboratory, there is a very high chance that these parts were factory tinned with leaded solder (at least here in the USA.) (Maybe it’s time to start ordering lead-free components.)
Soldering with lead-free solder to tin-lead solder-tinned component is a bad idea. It is difficult, extremely time-consuming, and you will not be satisified with the result. Put your component into your favorite ‘helping hand’ fixture and go at it. Use far more solder than you ever have tinning a lead. Let the excess drip off, or encourage it to cling to your lead-free solder iron. Doing that will dilute the tin-lead solder. Then when you place the through-hole component on the board, you will find it quite easy and quick to solder securely.
So make it a habit to tin EVERY through-hole part with lead-free solder. It is quick and easy if you are using rosin core lead free solder. I am using tin-bismuth-silver solder.