It is pertinent to note that we should thread our Yagnyopavitam on our own while threading one must keep oneself and the Yagnypoavitam pure. There are books regarding the invoking of a sacred thread or else one can take the advice of their elders.
The mantras must be chanted appropriately to invoke the concerned devata-s, appeal to Perumal, and chant the Gayatri mantra for invocation.