Look up the word in Cambridge Dictionary and find the pronunciation in slashes, like /pen/.
Read each IPA symbol as one sound, then say the word. The mark ˈ shows the strongest stress, and ˌ a weaker one.
Say /ˈsɪs.təm/ aloud: stress the first syllable, SIS, then the softer tem: system.
Read the entry, step by step
If the symbols in an entry look unfamiliar, open the dictionary's phonetic symbols help page first, or tap an audio icon to hear the word. Keep that help page as your standing fallback for any new symbol.
Find the pronunciation in slashes, for example /pen/. Cambridge Dictionary writes IPA symbols between slashes, and the UK and US lines can differ, so choose your variety before reading.
Read each symbol as one sound, not a letter. /pen/ is three sounds, p, e, n. Say them slowly, then join them into the whole word.
Notice the dots and stress marks. The dot /./ shows a syllable break, as in /ˈsɪs.təm/ for system, though some Cambridge American dictionaries use a raised dot /·/. The mark ˈ shows the strongest syllable and ˌ a slightly weaker one.
Say the word three times, keeping the stress where the marks put it, and check the symbols help page again if anything still looks unclear.
Worked example
Here is a finished reading of a full entry, using system. The transcription is /ˈsɪs.təm/: six sound symbols, s, ɪ, s, t, ə, m, plus the dot and the stress mark, so read it piece by piece before saying the whole word.
The dot splits system into two syllables: /ˈsɪs/ and /təm/. The ˈ before the first syllable tells me that part carries the strongest stress, so I say SIS, then the weaker tem: system.
Notice what this finished reading shows. The dot did syllable division, the ˈ fixed the stress, and each IPA symbol stood for exactly one sound, so /ˈsɪs.təm/ became system in one pass.
Try the same reading on a new word, for example /ˌretˈel/ for retell. The ˌ marks a slightly weaker stress, and ˈ shows the strongest one, so say the second syllable with more force than the first.
Completion check
You can read the transcription aloud from the slashes to the word end, with the stress where ˈ and ˌ put them. That is the core task complete.
Click the UK or US audio icon to hear the word after saying it yourself, and compare your stress with the recording.
If any symbol still looks unfamiliar, the dictionary's phonetic symbols help page explains each one it uses.