Shulchan Arukh Even HaEzer · Siman 32

English

With a document how [is she engaged]? He writes to her on a paper or on a pottery shard even though it is not worth a Perutah "Behold you are engaged to me" and he gives it to her in front of witnesses. And he must write it for the woman he is getting engaged to like with a Get. If he didn't write it for her name, she is not engaged to him. He can only write it by her will, but if he wrote it against her will, she is questionably engaged.

English

Even if the document is invalid to use for engagement they evaluate the paper, if it was worth a perutah she is engaged but if not she is questionably engaged.

English

If she were a minor or a young girl [between 12 and 12.5 years old] and she is getting engaged through her father with a document her writes "your daughter is betrothed to me".

English

There are those who say that he must write the names of the man and the woman in the document of engagement the way he must write them in a Get. [Rema: If he wrote it on something attached to the ground or on something from which benefit is forbidden there are those who say it is invalid (Maggid Misheh Chapter 3 of Ishut), and there are those who permit it (Rashba 703). He does not have to write the time in the document of engagement (Rabeinu Yerucham). If he wrote it in his hand and there are no witnesses on it she is questionably engaged (Nimukei Yosef the Chapter of Four Brothers). If he gave her a document and said, on the condition that the paper remains mine she is not engaged (Ran First Chapter of Kiddushin).