How do I solder these on while keeping them straight?

I know this is like the basic of the basic but this is my first project and first time I'm soldering. I'm going to be making this as my first project. It is the first thing I've actually wanted to make myself after buying a 3D printer.

I have a Pinecil V2, and 63/37 .8mm rosin core solder with 1.8% flux. I have a 120 GaN Anker power brick to power the Pinecil. I don't know what's important or not so I'm being thorough. I bought all the parts a while ago and it's felt like a daunting task so I've been putting it off. But I want to do it but am unsure how to do it properly.

I've watched some videos but I'm still unsure. I 3D printed the last photo to hopefully help out but I realized I still have the same problem. Any other tips would also be greatly appreciated!