Question: What differences are there in serving a Low Mass of Requiem?
Answer: If you know how to serve Low Mass, it should be easy to adapt to a Requiem. There are a few omissions that you should be aware of in advance.
Essentially, prayers and ceremonies that have a particular reference to the living, or to rejoicing, are left out. For example, there is no Alleluia or Gloria Patri, and no blessing of water at the offertory, or of the people at the end.
1. There is no psalm Judica me at the start; the priest moves straight from the versicle and response 'Introibo ad altare Dei. Ad Deum qui laetificat juventutem meam.' to 'Adjutorium nostrum.' The psalm is regarded as having some character of rejoicing, and is also omitted in Passiontide.
2. The priest doesn't cross himself at the Introit, so the server doesn't either. Instead the priest makes a cross over the book. Notice also that the Introit has no Gloria Patri, since this has some element of rejoicing.
3. Check that the priest is saying the Dies Irae, since this is optional in the standard daily low mass for the dead. If so, then you don't need to rise to move the book until towards the end of it. If not, then rise at the start of the Tract 'Absolve' as usual.
4. The water isn't blessed at the offertory.
5. If you usually kiss the cruets, then don't at a Requiem. (In fact, I'd suggest you don't ever do it!)
6. The reply to 'Requiescant in pace' at the end is 'Amen'. This takes the place of 'Ite missa est' or 'Benedicamus Domino' in a Requiem.
7. Don't kneel for the blessing, since there isn't one. Move straight after the Requiescant to your place for the Last Gospel.