Those black emery boards sold in High Street chemists, such as Superdrug, can be used on small areas of jewellery. They can be customised to fit with scissors or a craft knife and bits cut off when worn out. They will last for ages.
Waterproof pens do not interfere with soldering, so they can be used to mark crosses or circles for locating parts to be added. They can also be used for colour coding solder strips.
To check that small objects are square, forget engineer's squares and go for graph paper. You will be much more accurate.
To divide a ring into four, fit it into the best match on a circle template from a stationery shop and use the four marks on the template. To find the centre of a disk, join the marks to make a cross.