Watch my YouTube-video about the topic first, to get a first impression ot the minimap-feature:

Now you're prepared to create a minimap in Smile Game Builder.

What materials and other stuff do you need:
- A map with the dimension of 30by30
- A minimap-image (.png) // import it first in SGB
- A dot-image for the player position [herodot] (.png) // import it first in SGB
- A dot-image for the quest marker (.png // import it first in SGB
- Three events to place the images on the right position [Event A] [Event B] [Event C]

optional:
- An event with a Quest-NPC, a Check-Quest-is-on-event and another event with a Quest-target.

First things first - how does it work?
[Event A] You have two pictures: one minimap-image (which gets the image-number "0" and the coords 700,305) and a herodot-image (which gets the image-number 1). You will call the images via a 'Triggered automatically (1 time only)'-event.

[Event B] In another parallel process event (Automatically start (will synchronize and run repeatedly)) you will use the Advanced Variable Box Op. to Assagin the Player Y Coord. in a Variable (1) and also the Player X Coord. in another Variable (2). [Event B]

[Event C] In a third event (Automatically start (will synchronize and run repeatedly)) you will check the variable value of the Playerxpos / Playerypos variable.
To make It easy you can start with a 4-square-grid of the map:
top left, top right, bottom left, bottom right.

So you can check if the player is in the top left very easily:
If the Players y position value is below 15 && Playerx pos. value is below 15 the playerdotimage should be placed in the top left corner of the minimap (you could do this with the Move displayed image-function).In my case the coords are 755 (x) and 372 (y).
Now you have to to the same thing for the other three cases (playerpos is top right, bottom left, bottom right). For my example game I use a map with the dimension of 30by30.
Now you can playtest your minimap-'game'.

Get a better result
After you have playtest the minimap-feature, you can go on and do some detailwork. For example you could use a grid with more areas than only 4 like in the example above. As a result the postion is more precisely. In my opinion a grid with 16 areas should work fine.
'1' - 2 - 3 - 4
5 - 6 - 7 - 8
9 - 10 - 11 - 12
13 - 14 - 15 - *16*
So in this example '1' is the top left corner of the minimap/game-map, and *16* is the bottom right corner.

Some sure you could add a questmarker on the minimap if you want, so the player could see where to find the Quest-target
Just place a Quest-NPC on the map and turn a switch on, if the player activate the quest.

Then place another event (Automatically start (will synchronize and run repeatedly)) on the map which checks if the switch is on. If so a questmaker-image (picture-number 2) will be displayed on the minimap.

After the player finished the quest - e.g. has delivered the Quest-item to the Quest-NPC - the switch should be deavtived and the image (picturel-number 2) will disapper with the hide image function.