It's personal preference, but I add my nicotine to the DIY bottles first and then cap them. That way I can close up the bigger nic bottle of 100mg nic and reduce the possibility of tipping/spilling/contamination.
I usually do six 50ml bottles of DIY juice in a session. They are mostly singe flavor juices, so I feel comfortable doing larger quantities. Once the nic is in the small bottles I can add PG, VG, and flavorings to each bottle on the scale and cap them. I make my bottle labels with a Brother label maker as I go and put the label on the bottle once the ingredients are added. My labels include all ingredients and percentages for future reference. Then I shake and set aside in the completed area.
I usually make a 50ml bottle of unflavored nic/pg/vg when I DIY. It's a good vape when you tire of flavored mixes and it's also useful when a mix is just more flavored than I like. When that happens I just add a little unflavored to dilute the flavoring without affecting the nic, pg,and vg proportions of the mix.