I did some electronic soldering in my teens and in college. I've occasionally done it since then to do minor repairs like replace part of a mouse I liked when it wore out after years of clicking. This past week I discovered that one of the spigots on the outside of my house had failed over the winter. The spigot (sorry, "spillcock") and the piece it screwed into had both been damaged by ice and caused a leak inside the wall. I took a picture of the joint and accompanying piping and sent it to my dad for advice. He was pretty horrified at the quality of the surrounding joints, so he strongly suggested replacing more than just the two parts that were causing the leak.

I went out and bought some lead-free solder and flux, but didn't think to look up anything about flux or solder quality. I was having trouble getting a joint I was happy with, until I finally gave in and asked my neighbor (a plumber) for some advice. He give me some very useful advice (give the excess flux a wipe once it starts bubbling, start applying solder at the bottom to let steam escape out the top of the joint and then work your way up, among others). He also gave me his favorite (lead-free) solder and flux to use. The flux looked really grody, and unsurprisingly made a huge difference in how well everything flowed. In the end, I replaced the suspicious section with my much less janky piece and it's now dry as a bone.

To add to that, you add the solder to the heated part and it will flow into the point you're soldering. Don't try to transfer it from the iron or paste it on the spot semi-melted.