if you're not experienced at it, i would caution you NOT to go directly ventral mid line (exact middle of the belly) as there are blood vessels there. i've seen alot of people do that without harm but i'm too jumpy to do it myself that way. go a bit lateral and flip the scale of course. but i think the most important part is to do it sooner than later when you can still aspirate out much of the yolk before it solidifies. if it doesn't cost too much let the vet do it under ultrasound guidance.