I suggest that the choice of items here, cedar, scarlet yarn, bird and hyssop is both practical and culticly significant. I will not comment on the cultic significance but allow me to offer a simple comment about the practical side of this.
First, not that we have scarlet yarn/thread specifically mentioned, along with a piece of wood and a sprig of hyssop. The priest was to told to dip "them" (אֹתָ֗ם Lev 14:51) in the blood and water.
By this I assume that the priest used the scarlet yarn to tie the hyssop and bird to the cedar to temporarily make a single object to sprinkle the blood and water. Following the seven sprinkles, the unharmed bird was to be liberated (V53), presumably by cutting the scarlet yarn.
A similar procedure was used in Lev 14:6 where Ellicott observes:
(6) And shall dip them and the living bird.—With the crimson thread
the priest tied together lengthwise the bundle of hyssop and the cedar
wood, extended about them the wings and the tail of the living bird,
and then dipped all the four in the mixture of blood and water which
was in the earthen vessel.
Gill provides more detail:
and the cedar wood, and the scarlet, and the hyssop; which were all
bound up in one bundle, but whether the living bird was joined to them
is a question; according to Jarchi they were separate, the bird by
itself, and the cedar wood, &c. by themselves; they were neither bound
together nor dipped together; and Ben Gersom is very distinct and
expressive; we learn from hence, says he, that three were bound up in
one bundle, but the living bird was not comprehended in that bundle;
but according to the Misnah (c) they were all joined together, for
there it is said, he (the priest) takes the cedar wood, scarlet, and
hyssop, and rolls them up with the rest of the scarlet thread, and
joins to them the extreme parts of the wings and of the tail of the
second bird and dips them; and this seems best to agree with the text,
as follows:
and shall dip them and the living bird in the blood of the bird that
was killed over the running water; that is, into the blood of it as
mixed with the running water in the earthen vessel, which together
made a sufficient quantity for all these to be dipped into it; whether
separately, first the living bird, and then the cedar wood, and
scarlet, and hyssop, or all together: the bird that was kept alive was
a type of Christ, who as a divine Person always alive, and ever will;
he is the living God, and impassable: the dipping of this living bird
in the blood of the slain one denotes the union of the two natures in
Christ, divine and human, and which union remained at the death of
Christ; and also shows that the virtue of Christ's blood arises from
his being the living God: the dipping of the cedar wood, scarlet, and
hyssop, into the same blood, signifies the exercise of the several
graces of the Spirit upon Christ, as crucified and slain, and their
dealing with his blood for pardon and cleansing, as faith and hope do,
and from whence love receives fresh ardour and rigour.
