Many words will be automatically corrected to include accents, so there is no need to do anything special.
To manually add accents to your words, long press (press and hold) on the letter and choose the accented character by sliding your finger to the letter of your choice.
SwiftKey automatically detects which accents are used in the languages you have enabled. If you wish to enable all accented characters:
- Open the SwiftKey Hub
- Tap 'Keys'
- Check ‘Accented Characters'