Jenny,
When putting the counter and to box in I usually last the shoe, release the upper and finish the lining and glue it down.
Now I fold back the upper and put on the toe box, not pre-formed but it goes on moist with glue. After the toe box dries I scrape it and finish it very smooth, any unevenness here is going to show like a big spot when you finish the shoe, put on my side linings and start with the heel counter, The heel counter goes in moist as well. Now I fold back the upper and finish lasting. The heel counter is moist with glue on both sides. I work it with my hammer to make sure it is totally smooth, any unevenness is worked out with my hammer.
Let the counter dry before finishing the rest of the shoe if you cement unless you inseam, you want to work it as the counter is still moist.
A different way to do this is put the moist counter and toe box in between the upper and lining. Now last the shoe and try to keep the counter and toe box from slipping out, they are very slippery little devils at this point.
Last the shoe and work the leather with your hammer to get it very smooth. If you cement let it dry or the cement won't stick. This technique is much quicker but requires much more practice and the results would in my humble opinion be a little less perfect.
Pre-forming counters is not the way to go, at least not in my not so humble opinion

. Works in the shoe industry for high volume production but you do loose something with it. Take the counter v/s heel lining. You typically have a seam in the heel lining, a leather counter formed wet over the lining will adapt to the unevenness and make it virtually undetectable, if you pre-mold the counter and feel the seam you can feel the thickness of the underlaying leather. Just my

Don't hesitate to try it both ways and tell me if I'm wrong.
Hope this helps you along
Rob