Keep it simple. Make a note of your start time, I like to keep my start times to 15 minute intervals so 06:00am or 06:15 etc as its easier to keep a track of things.
Lets say you started at 06:00am. Now by 12pm midday you need to of taken at least a 15 minute break, by 3pm you need of taken another break of a minimum of 15 minutes. That should cover you as your shift time as gone past the 6 hour cut-off point. By 6pm you will have gone past the 12 hour mark and you will need to of taken another 15 minute break minimum.
At our place, our tacho software will flag you if you work more than 6 hours without a break so in my eyes, if I take at least a 15 minute break every 6 hours then I should be ok.
Now, your driving breaks should cover you for WTD most of the time. For example, I do a run to Newcastle on a Monday morning. I start at 3am for a 7am tip in Newcastle city centre. By the time I’m done there, I’ve already done over 3 and a half hours driving, getting on for 4 hours and I’m getting very close to the 6 hour mark on my shift so I know I need to take a break. My 45 minute break to reset my drive time will always count towards the WTD.
Of course, its not always that simple but the more you think about it, the easier it will become.