Work with the fields that ToodleDo has
First, a disclaimer. I'm a pragmatist, not a GTD afficionado. I acknowledge that I need reminders for a lot of regular tasks that I do in an interrupt-driven life, so I use them. I've not had great experiences in the past with apps that use Calendar integration for date/time-related tasks, especially for repeating tasks with alarms, of which I have many. I find that it works better to have all my tasks held within my ToDo umbrella, rather than having things in a separate calendar (even if integrated). And I REALLY want to be able to manage my tasks on a SmartPhone.
Personally, I'm inclined to use the ToodleDo fields for the uses they're intended for as far as possible. I suspect that some of the GTD implementations that people have recommended were set up when ToodleDo's choice of fields was more restrictive. I may be wrong here, as I've recently returned to ToodleDo after an absence, so I'm assuming that things have changed during that time.
ToodleDo now has fields for Context, Location, Status (None, Next Action, Active, Planning, Delegated, Waiting, Hold, Someday, Cancelled and Reference), Star, Start Date and time, Due Date and time, Goals, Length, and a couple of others. The paid version supports one level of subtask - for projects in the GTD sense, i.e. those tasks that have more than one step (but that are not more complex Projects with a capital "P").
I use Folders for Areas of Responsibility, and Tags for Big Projects with a Capital "P", but some people find that it works better for them to have these the other way around. I guess it depends on whether you want to consider Folders as another hierarchy level to group tasks and subtasks together and something that can be finished and archived.
I have a Saved Search for my Focus tasks, which is defined as Status is Next Action / Active / None (in case of anything I added quickly without setting a Status) AND is either Starred (i.e. I've manually marked it as something to focus on) OR is due today. You may want to modify this to include/exclude whatever Status options are appropriate for your usage. Or you might exclude the Status part altogether. In theory it should be redundant anyway ;-).
Similarly, you can create Saved Searches for your review list (probably based on Status, plus due dates for stuff due "soon").
You can slice and dice by context, location, duration (if you use it), etc. - and create Saved Searches for those too, if you want to refine your view based on combinations of fields.
One of ToodleDo's strengths is that it allows you to choose which fields you use, and remove the unused ones from view altogether, so that your desktop isn't cluttered with columns that you don't care about.
The main thing for you to do is to identify how to make the GTD principles work best for you, decide which fields you need to do that, and use them in a way that works for you.
If you find that ToodleDo doesn't have a field for something you'd like to use, you can either hijack something you don't need, or use Tags. For instance, ToodleDo doesn't have a field for energy levels (to help you decide which tasks to do according to how you feel). If this is something you like to use, one way to do it is to use Tags, optionally with a convention for energy-related Tags (you may find it helps if they're prefixed or suffixed by some consistent symbol or characters). A task can have as many Tags as you like, so you can use energy-level tags as well as tags used for other things (Areas of Focus, or Projects, for instance).
Alternatively, you could use another field to denote energy levels instead (e.g. Priority or Length, if you're not using them for their intended purpose). The potential advantage of using another field is the ability to sort by the value of that field, rather than just searching for a particular value. You can't do this with tags, because you can have many tags per task in any order. However, if you use a field that's intended for something else, check what features ToodleDo offers in connection with that field before deciding whether it will be a good fit. For instance, Priority has an impact on the Hotlist, and can be used to hide tasks with negative priority. This may be useful to you, or a hindrance.
If you want to sync with a smartphone app to manage your tasks, some of your decisions may be influenced by what apps are available, and what features they support. I find that I use mine a LOT (because I almost always have it with me), and the ToodleDo website surprisingly much less than a QWERTY keyboard fan like me might expect.
At the moment, I don't use Location alerts on my phone, because the app I use only uses GPS for location, which is a big battery drain. But when that's been resolved, I look forward to being reminded about stuff when I'm in the right place. For now, I content myself with using the location field to manually check for tasks to do when I'm there.
