The t is fully pronounced and the s keeps the /z/ of plain was; in UK speech the second syllable reduces to schwa /ənt/.
The apostrophe replaces the o of not between n and t. Wasnt is nonstandard, and was'nt puts the apostrophe in the wrong spot.
In a sentence
wasn't /ˈwɒz.ənt/ (UK), /ˈwʌz.ənt/ (US): the contraction of was not.
He wasn't at the meeting this morning. The contraction stands in for was not, and the apostrophe marks exactly where the o dropped out.
I wasn't sure what you meant. In UK speech that reads as /ˈwɒz.ənt/, in US speech as /ˈwʌz.ənt/. The n't is one spoken unit, not a separate n and t clash.
The transcriptions make the details easy to spot. The t in wasn't is still fully pronounced, and the s keeps the /z/ sound of plain was. What changes is the second syllable: in UK speech it reduces to schwa /ənt/, so the vowel you actually hear is schwa, not the a printed on the page.
The apostrophe replaces the omitted o of not, so it sits between n and t. Was'nt places it before the n, which is the wrong spot, and wasnt without any apostrophe is nonstandard in careful writing.
Spellings like wadn't or warn't turn up as pronunciation or eye-dialect forms in fiction, but wasn't is the standard written choice.
Apostrophe and spelling traps
As the transcriptions /ˈwɒz.ənt/ (UK) and /ˈwʌz.ənt/ (US) show, the t in wasn't is fully pronounced and the s keeps the /z/ of plain was. In connected UK speech the second syllable reduces to /ənt/, so the vowel you hear there is schwa, not the a printed on the page. That reduction is worth keeping in mind when writing: the spelling is fixed, so an a you barely hear in speech still has to be written.
The apostrophe replaces the omitted o of not, so it goes between n and t: wasn't. The misspelling was'nt places the apostrophe before the n, which is the wrong spot, and wasnt with no apostrophe at all is nonstandard in careful writing.
Some printed texts, especially fiction, show wadn't or warn't as pronunciation or eye-dialect spellings that mimic casual speech. These belong to a character's voice, not to careful writing. In anything else, spell it wasn't.