I mixed up a sachet of IG as per instructions, using cooled boiled water. Then when DS seemed hungry, I would give him 5 mls in a syringe (squirt it quickly and gently into the inside of his cheek, not the back of his throat) before feeding, and another 5 mls when he'd finished. If I could. If he was asleep on the boob, I didn't wake him up to do it.
I tried giving a little before as sometimes I felt giving him IG when he was full after a feed was a bit distressing for him, and could cause a vomit, when we were trying to stop him puking.
After the first couple of days, I never gave the full dose of IG per 'feed.' For a start, DS was a snacker, so exactly what constituted a 'feed' I didn't know. It wasn't as if he was getting a measured bottle at timed intervals. If I gave him 5 mls and he only had a short feed, then I didn't feel like I'd shovelled a huge dose of IG down to cope with not very much milk. Also, it sorted his reflux but gave him awful constipation... so I reduced to dose to control the sicking, but without bunging him up.
Any chemist should sell you those small 5 ml syringes - much easier to dose medicine with than a spoon, or having to put it in a bottle.