What does the Eruv Tavshilin consist of?
Discussion:
The most preferred manner of making an Eruv Tavshilin is to use one cooked item and one baked item (Shulchan Aruch 527). If one only used a cooked item the Eruv is still valid. However, if one only used a baked item, the Eruv is invalid (M.B. 527:6-7). In such a situation a rav should be consulted as there are ways around this issue.
The Cooked Item:
The cooked item should be something which is normally eaten with bread i.e. fish, meat, poultry etc (M.B. 527:11). One should use a nice portion since it is being used for a mitzvah. However, one must use at least an olive size portion. It is best if the cooked item was cooked specifically for the Eruv (527:6 Beiur Halacha ibid).
The Baked Item:
One should use bread or matzah for the Eruv. The minimum amount one needs is the size of an egg (Rema 527:3). It is preferable to use a nice sized portion for the baked item as well.
What does one do with the Eruv?
The Eruv must remain intact inorder to permit melacha for Shabbos. If one lost his Eruv or it was eaten then a rav should be consulted. If part of the Eruv was lost but at least an olive size remains that is sufficent (S.A 527:17). The Eruv should be eaten on Shabbos, some have the custom of using the bread for the lechem mishneh by each meal and eating it at the third meal (M.B. 527:11).
If one forgot to make an Eruv Tavshilin a rav should be consulted