Is it possible to wire my new accessories into the existing fuse boxes. Is it just a matter of clamping the wire into the box, or does it require soldering?

I need to wire:
one yellow, always supplied with power regardless of ignition switch position and

one red, controlled by ignition switch on/off.

I also need help deciding what size fuses I should use. I'll have to look up how much power it consumes.

 

I hope I posted this in the right forum, if anyone thinks that this should be posted in a different category, please let me know, thanks