The sounds are not difficult for Spanish speakers, but we do tend to make mistakes with individual words, especially with / ð /.
The main difficulty, though, may be recognising the sounds, as you may find the phonetic symbols intimidating.
- / θ / is pronounced like the Spanish "z".
- / ð / is articulated exactly the same as / θ /, the only difference between them being that / θ / is voiceless, "sorda", with no vibration of the vocal chords and / ð / is voiced, "sonora", so it is pronounced with vibration of the vocal chords.
