I have used this technique in the past with good results.
Get a large shot needle and have it in camp. Needle should have about a 1 /32 inch opening. Mix borax and salt with water. Liquid should a little be milky. Hang the antlers upside down, and using the needle make a nmber of holes in each antler tine. Make sure you get each tip.
Here is the tough part. Feel just above the burr for small dips or canyons in the antler. These are the blood vessels that supply the velvet. Insert the needle into the small reverse ridge and inject the solution. You have to go along the antler not directly in. You are finding a vein just like a doctor, nurse, or paramedic, and unfortunately a number of other people in different situations do. Leave the needle inserted and unscrew the back refill and inject more solution. The solution will force the blood out of the antler through the holes you made in the tips. When you hit the vein right you will know it because blood will just run out the tips.
You will have to find a vein in each antler tine. You do this by just working you way down the antlers. Keep forcing the borax and salt solution in until you don't see anymore blood coming out.
Hang upside down and let dry. After a couple of hours I use a rag to gently rub down the velvet, forcing what ever liquid is left out of the antler. Then keep them as dry and cool as you can. This is a lot of work, but will save the velvet.