This is not so much a tutorial as a suggestion or alternative. A lot of older machines are missing slide plates and bobbin case covers. If the plate is square, I suggest plexiglass.
Plexiglass can be purchased at most big-box hardware stores in sheets. I measure the plate with a dial caliper, lay it out with a carpenter's square, and cheat just a hair on the large side. Marking is done with an awl or a scribe, and I prefer to cut it with a fine-tooth coping saw. Many machines use plates that are angled (beveled) on the edge, so you can either saw on an angle or trim it later with a sharp knife. This will allow the plate to slide into place without falling off. Any knife marks are sanded out with fine-grit sandpaper.
I like plexiglass for several reasons. First, it is relatively cheap; you can buy a window-sized sheet for the price a replacement plate costs on ebay. Second, it is clear and scratch-resistent, which means you can often watch your machine working as you stitch. Third, it is rigid and relatively easy to work, unlike plastic and a lot of metal alternatives. And fourth, it won't rust or tarnish (although it will scratch if you try hard enough.
I made plates recently for a Singer 27, a White Family Rotary, and a Singer 66. The 66 was missing the plate retaining spring, otherwise it would have been a lot harder.
Hope this idea helps someone with a missing plate!