I've done most of those things at least once. If you don't paper piece often, the solution might be to make a practice block first. As for placing the triangles the wrong way, I place them in the finished position first, then flip to the sewing position. For trimming the finished block I place the 1/4" line of the ruler on the stitching line, ignoring the cutting line. After doing a lot of them you will get in the habit of doing it everytime and will save your seam allowance. Sometimes after positioning the fabric I pin it in the seam allowance, then fold on the sewing line to make sure it will cover the area. And mostly I sew with the fabric on top so I can see what I'm doing all of the time, and I only have to trim the seam allowance after it is sewn, not before. You need extra lines on the paper to do it that way. I don't think Margo will be doing a tutorial on this but if you've ever sewn half-square triangles together without marking and using a line extending from the needle to guide the fabric, it is similar to the way I paper piece. Not all blocks will work this way.