>>13194
Where did you hear that?
While on the first glance it seems much more polite to pour first and then drink, most sacrificial rites that survived christianisation used leftovers for such purposes without being thought of as disrespectful as the christian peasants still did not want to anger the minor spirits they still believed in until the industrialisation.
Now, for a libation in the latin context you might want to look after hellenistic sources, as there are probably some pretty detailed ones which could be simply used for whatever deity/rite you chose as you still use a framework of an indoeuropean religion which actually syncretisized in the past in the rhineland making this a combination that is historically legit. (you had germanics there pettioning romanstyled altars for mercurius-wodan)
You are probably searching for something decidely germanic to answer your question rather then using roman methods so I tried looking for historic informations.
Of these, only those written by christians could be found, looked mainly into a very early story of the 5th century was in which the description of the beer sacrifiece to wodan/Odin was observed by saint Gallus who then proceded to blow the barrel used for the rites into pieces, smash some idols and convert the villages of the alps.
The process itself was only mentioned to highlight the saints deeds and no direct description could be found, however some interesting commentary by the academic source which had the original latin text of St. Gallus deeds for educational purposes:
http://www.bg-gallus.at/fileadmin/dateien/latein/4_Textus/text_gall_brigantium.pdf
"Quo cum moraretur et inter habitatores loci illius progrederetur, repperit eos sacrificium profanum litare velle: vasque magnum, quem
vulgo cupam vocant, qui XX modia amplius minusve capiebat, cervisa plenum in medio positum (est). Ad quem vir Dei accessit
sciscitaturque, quid de illo fieri velint. Illi aiunt se Deo suo Vodano nomine, quem Mercurium, ut alii aiunt, autumnant, velle litare. Ille
pestiferum opus audiens, vas insufflat, miroque modo vas cum fragore dissolvitur et per frustra dividitur, visque rapida cum ligore
cervisae prorumpit. "
(Only describes the amount of beer, the god invoked and that it was placed in the peoples midst-duh)
to which was added:
"cervisa plenum> This drink-sacrifiece was not like the libatio as practised by the romans, but a cultic unification/union with the god: "At the beginning of a pagan drink-festival the liquid was sanctified in the god's name and thus energised with its essence, Wodan/Odin himself was present in the drink
Sadly no further source for this remarkble statement was given (a polytheist eucharist?) and google did not spat out anything either. This being a community right might be important-a south germanic equivalent to the sumte described in the edda? In that case it wouldnt be relevant to your problem as a personal sacrifiece is different to the rigid community rite which is a whole nother topic.
As that one claim was unsourced, a germanist of the 20th century who wrote a whole book concerning sacrificial practices that could still be observed or reconstructed in scandinavia and continental germany could have helped, but while the stuff he writes about is highly interesting, the order of sacrificing and drinking yourself does not seem to be that important, in some cases he interpretes the in early medieval times widespread and sometimes by the church sanctioned drinks for saints at certain festive days in a way that the act of drinking itself in a certain mood was considered holy as the liquid was considered blessed at these days and times.
https://archive.org/stream/germanistischeab03bresuoft#page/274/mode/1up/search/trankopfer
Also some general stuff about alcohol and european cultures, skipping over it might be interesting as various kinds of libations get mentioned albeit its mostly off topic.
http://scholar.uwindsor.ca/cgi/viewcontent.cgi?article=1025&context=llcpub
Id say, whether you give what is left or the first sip is less important whether you do it mindfully and belief that you commute with.. something.. while doing it.