Are you talking about the vertical section that comes up from the inner wing to the box rail that the outer wing bolts to? This part generally available as a repair panel. The box and the top of the vertical part rusts out. The curved inner panel that goes down to the chassis member usually survives well and shouldn't require replacement. If replacing the vertical part the position and alignment is done using the hood. Make sure that is correctly positioned, especially at the back where it needs to fit well. Match it to the front panel. This might leave the current wing position showing uneven gaps but the new ones will fit better. Do one side at a time and tack weld them in to check their positions before welding up. The back corners can be difficult. Beware sparks on the glass, cover the screen, and remove anything inside that can burn.

Have a browse around the pics here on this and other projects.
http://amazon.forum.bilia.se/phpbb3/viewtopic.php?f=13&t=7591&start=30