When is a next action not a next action

[mesmer]

I have just read the book and listened to CD's and i am possibly reading too much into it but im having a problem grasping this concept. I have searched the site and cant seem to find an answer so please treat me gently the answer is probably staring me straight in the face but here goes,

Sorry to go on about this;

i have read other posts but still am a little confused ( a lot actually ). If i have a project list say .....buy car ? the next action for want of a better suggestion would be in simplistic terms.....

(i am sitting at my desk in front of my computer with the phone at my hand)

1 Identify what make and model i want (from desk/computer)
2 Phone around for best price on that model
3 phone for insurance quote get best deal
4 phone to Make best deal with salesman to drive away today
5 Buy car

over simplified i know but my point is when do you stop doing the items that appear to be next actions before you finish the project....
I can see certain projects that can be started and finished in one go as each action becomes a next action after the previous next action gets done and then the another item will become the next action to move that project along so i do it and so on untill the project is completed .

Am i missing something ?

in the above sittuation i am in the right place with interenet and phone at hand to complete all the items in the project/list.

i hope ive explained myself clearly (semi clearly ? )

Please no sarchastic trplies im struggling enough as it is without being knocked back even further.
Many thanks for any advice you can offer this idiat ! or even idiot

Mike