Good question.  There is not a simple answer, either.  The basics are this:

1. Do the export
2. Transfer the file to your laptop/workstation
3. Edit the file in notepad and save it
4. Transfer the edited file to the new system
5. Run "/import file=FILENAME" in a terminal window

The only part of this that needs expansion is number 3.

When I edit these files, here are the things that I do:

1. In the /interface ethernet portion, you need to remove the excess "fluff" from this line.  If the interface name is not at it's default value (ether1, ether2, etc.) you have to account for that. Here are a couple of examples (they've already been edited down):

/ interface ethernet

set ether1 name="Public_ether1" auto-negotiation=no \
    full-duplex=yes speed=100Mbps disabled=no

set ether2 name="LAN_ether2"

These 2 entries show a couple of interfaces that have had their name changed and one of them has turned off auto-negotiation.  All the other settings that were in the file for these ports were default settings (mac-address, mtu, etc.) and were removed from the backup. 
(NOTE: If you leave all the other settings there, everything will work as expected, BUT YOU MUST REMOVE THE REFERENCES TO THE MAC ADDRESS!)

2. Go through the file and remove the settings that are default. For example, you can generally remove these (and some other) sections entirely:

/interface wireless align
/interface wireless snooper
/tool e-mail
/tool sniffer

There are many sections like that, that are in the export, but are not needed to move a config from one router to another

3. Check the /queue simple and /queue tree and remove some of their excess settings.  This takes a LOT of experimenting to get it right if you are not familiar with what parts are needed and which are "fluff".

My experience is that a simple config will be only 15-30 lines after it is edited.  Of course, if you have firewalls or queues it will be longer.