I'm not really a fan of the ones on YouTube as they are usually only using 1 compound and it's usually not MCT oil and they hold the needle vertical.
When I blast I inject Ed so;
Step 1: Grab 7 slin pins
Step 2: Grab 1"back filling" syrgine for every compound im using
Step 3: draw weeks worth of each compound in each "back filling" syringe
Step 4: pull plunger out of slin pin put back in wrapper
Step 5: hold slin pin as horizontal as possible so oil won't reach the needle needle.and inject largest volume compound first and do on so forth
Stes 6: replace plunger untill I feel resistance then tip slin pin vertically with needle up wait for oil to drain back against the plunger then push in rest of the way.
Step 7: repeat step 4-6
The reason why I don't do all 7 at once and just rest them vertical on their caps is BC the oil (especially MCT) leaks out the needle by the time I'm done filling all 7. (Lost a week's worth of deca first time I tried

)
if you wanted to save on filling syringes you could use the same one for each compound but after u backfill 1 slin pin u have to put the plunger back in far enough to cause an airlock so it won't leak but syringes are cheap so idgaf