The more you get into working on and setting up guitars, the more the proper tools become useful.
A notched straight edge is essential for getting the neck/fretboard level before fret levelling. You can make your own with a steel or aluminium strip and an angle grinder (or a file if you have the patience), though they aren’t that expensive to buy. It’s the flat edge that’s important, the gaps are just there to fit over the frets.
The ubiquitous orange-handled fret profile file that you get in most fret care kits from eBay or Amazon is OK, if a little rough (I found it left chatter marks on the frets) and the radius is a bit wide for normal frets. It’s also a bit deep if your frets aren’t that tall, so that you can get the sides sitting on the fretboard whilst the top of the radius is held away from the top of the frets. But one helped me profile some stainless steel frets before I got a better file.
Getting a file with the right width for your frets helps to get a good result. The slot should be the same width or just wider than the fret itself. All the files I’ve seen come in narrow (2mm), medium/standard (2.5mm) and wide (3mm) sizes. Also 1.5mm for smaller frets as used on mandolins. I’ve now got files for all of those, though a standard 2.5mm one should be fine for most guitars.
I find a radius block gives me a better result in setting up for a really low action than the straight fret levelling file I started out using, though that still had its uses.
A triangular file with at least one safe edge/corner is very useful for rounding the ends of frets to remove sharp edges.
I use wet and dry grits P240 up to P600 and then up the Micromesh grades to polish the frets. It’s important to remove any fore/aft grooves in the frets as these will make the frets feel ‘scratchy’ and prevent smooth string bending. After the P240 grit sand, I use a 5-6cm offcut from a top E string to run along each fret to make sure I can’t feel the string catching. If I can, more strokes with the P240.
I do the same number of rubs with each grade of paper to try and keep the frets level. If you had one badly lined fret, it might take 50 strokes to remove the lines. If you only did 10 on the other frets, then that fret could now sit lower than the rest.