The above video shows what I described in my previous post, the process takes less time one may think, grinding in a mortar is recommendable when one of the components tends to clump, otherwise isn't necessary.
In order to encapsulate properly the amount of powder necessary to fill up completely the capsule has to be established first, for example typically a 00 capsule holds about 500mg of powder, the exact amount may vary according to specific weight and particle size of the powder.
In the case one aim to encapsulate 5-10mg of active a filler is needed.
Chose your filler, it can be as simple as sugar, very fine sugar used for cakes toppings is very cheap, ready available, very fine...and no, in those amounts isn't likely to make you diabetic.
You can choose any other filling if sugar doesn't make you comfortable with, rice flour, maltodextrines, protein powder, etc...
Once you have got your chosen filler fill up 10 capsules with it (or what your encapsulating machine holds), pour out the filler from the capsules and weight it, divide it by the number of capsules and you'll know exactly how much filler each capsule holds, of course.
Now do your math and weight the amount of filler needed for the amount of capsules you want to make, do your math again and weight the amount of active you need for the same amount of capsules.
Now mix the filler and the active as described and shown in the video, the active in such tiny amounts isn't going to change much your calculations.
Once ready encapsulate the obtained powder filling up the capsules properly.
You can't think to encapsulate just 5mg of powder, it isn't going to work, not with many capsules at least (you might do it for one or very few ones, weighting each time the active with a precise scale but it is tedious and not a smart choice anyway), period.