this process took me two hours but this takes moments to explain. There are two possible ways to fill your markers. You could use the needle method to fill but this is not the way I do it. 

  
First I gather everything markers, refill packs, paper towels (minions aren’t required but would make this process so easier) scale, replacement nibs, and tools. 

   
   
I fill my blender pencils most often and I can’t imagine any using a color more often then blender. 

  
I have both sketch, square, and refill pictured above. That’s because the weights are different and that’s important for this style refill. 

First step – gather supplies. 

Second step – take off both caps

Third step – take the chisel nib out 

   
 
Use the tool and not your fingers if you don’t want to destroy your nibs and need to replace them. 

Fourth step – weight markers to see how much ink they need

   
Sketch weigh (on average) 9.4 grams with both lids off and chisel nib out. 

 The square markers weigh (on average) 14 grams with both tips off and chisel nib off. 

Anything below that and you might as well fill them up while you have it out. 

Fifth step – drip ink from the correct refill (and yes check to make sure cause you don’t wanna deal with the consequences of filling it with the wrong color). 

Try not to over fill cause it drops out the other end if you do. It’s slightly messy and might stain stuff (it’s why I do everything over paper towel). Don’t worry to much if you over fill. You can bleed off extra (it’ll just expensive and expensive and did I say expensive). 

If you drip on something try removing it with rubbing alcohol and hand sanitizer. Dark colors, red, and orange can be trickier. 

Make sure to clean your nib removal tool (tweezers) between markers. Transfer isn’t hard to bleed off (like filling with wrong color) but you might not realize you need to correct it until you’ve ruined some bit of art you’re working on. 

Good luck and have fun filling. (Oh and you might wanna have a book on tape or some music cause the more markers you have the longer this will take)