What would be an efficient way of penning down these branches with my own activities on paper?
The GTD book was written at a time when paper-based would be the norm, for at least many of the executives that it was directed towards. The flow does not need any computer or electronic means, which is why one very popular productivity blog took the name 43 Folders after the GTD idea of having that many physical folders to manage your items (across days of the month, and months of the year).
In part, your adoption will depend on if you want an electronic backed system, or a complete paper. For an electronic backed system, you'll need to add in steps of printing your various to-do lists to carry with you, for the various contexts you are in, and having paper-capture on the road. Once back at your computer, capturing those paper in-box items into your electronic workflow, and dealing with them.
Complete paper-based has a following. While the hipster pda seems to have started as an ironic effort, it does (did?) have a serious following. A google for that phrase turns up many hits like this and a follow-on, but basically they use index cards as single items, add a few blank cards to capture new things, add a bulldog clip to hold it, and you're good to go.
Became too long for a comment.
The GTD process diagram is not a way to deal with actions; rather, it is a process for deciding/differentiating stuff as being an action or not.
The Hipster PDA is a capture mechanism. It's nice because you don't need internet or electricity to maintain it.
What I would do, when I have to use something like it, is either group the cards together (context), use a card to group thing together (calls), or some combination.
Then, when you look at a card/action, you run it through the four step process of deciding what work to do. To reiterate, the GTD flow diagram is a separating/identification process - actions don't go through the process; they're already through it. The four questions for deciding what to do next are:
Using a color-coded system to note what to do next would increase busy-work and waste because you would have to take the "deferred" action from the pink index card and move it to the green "next actions" card, then throw the pink one away. Instead, I would recommend capturing everything on the index cards - in a grouping methodology that makes sense to you and you can scan through quickly - then, when you want to decide what to-do next, just go through the cards and ask yourself the four questions.
Hope that helps.