Richard Schneider has posted a guide on how to prepare certified translations in Germany: Von Schuppen und Ösen. It’s in German but with illustrations.
Here’s a summary of the main headings
1. The stamp and signature should be blue rather than black. (I definitely use blue stamp ink, but agree it would make sense to do the signature in blue biro too. Then people can see immediately whether the translation is an original or a photocopy).
2. A round stamp is not prescribed, but looks more official: official stamps are usually round, and rectangular stamps are easier to copy.
(In Bavaria, the round stamp is in fact prescribed. It can get quite expensive when you think how long the translator’s title is).
3. Always attach a copy of the original text. (I usually do that, though it’s not always feasible. The courts don’t want it for internal use. But it covers you if you translate from a copy rather than the original, because the recipient can compare the copy with the original and see if you translated the right document. And if you do a translation of excerpts, you can use a highlighter to mark on the copy which bits you did or omitted).
4. Fold the pages over so the corners are staggered (see photo) – each then gets some blue ink on it from your stamp.
5. Add a stamp on the fold inside.
6. Bind the pages together so they can’t be separated. Use an Öszange (see picture). This is apparently an eyeletter punch. Alternatively, you can use a paper seal (Siegelstern; see pictures). (I sometimes use gummy paper, which I cut otu in a rectangle, and put the stamp on top of it. And sometimes I sew, with needle and thread).
(Richard Schneider seems thrilled with his punch. So was I when I first got one. The problem came when I sent off a set of punched translations and they passed through a machine at Deutsche Post, which ripped the translations to pieces, and I had to do the whole lot again. I have never used the device since.)