The factory manual says to have the lower balI joint connected and the control arm pivot bolts out, and to put the spring in place and jack the LCA into place. That's sounds like the way the factory manual on the 82 Grand Prix (G-body) I had said to do it, only for the G-bodies they also show a J-tool jack adapter that no one actually has. Either way, that method sucks (I did it that way once). But what works well on the G-bodies - and maybe on Fieros - is to use a spring compressor like this one
but without the upper spring jaws (they slide off). Put a couple of flat washers under the bolt head and drop the bolt through the upper shock mount hole. Hold the spring in position, with the bolt through the center of it, and thread the lower jaws onto the bolt. Engage the jaws on the spring and compress it. Then swing the LCA up and attach the ball joint, and remove the spring compressor.
I haven't tried this on a Fiero - the springs look shorter than the G-body's and may also be smaller diameter.. This method assumes the compressor will fit as needed and compress the spring enough to do the job. But if it fits, the job is easy.