Type one Asterix * prior your text, and it will convert to bold. Tap on underscore _ and add it to any side of your text to get the italicised message. You can create strikethrough by adding a tilde ~ before or after your text.
See all the steps are soo easy. All it needs is a little bit of practice. Also Read: iPhone Users can Secretly Read WhatsApp Messages without Triggering Blue Ticks