As soon as you enter the room.

Have it sitting out in plain view, on a coffee table or night stand.  That way you don't even have to mention it.  She'll just pick it up when she's ready.

You should have the correct amount typically in an envolupe but always separate & ready.  If you go to her location, you place it as her web site directs.  If no direction, place it on a dresser or bedside table in plain view.  Do not mention it or look at it again.  
If she comes to you have it ready in a similar location.
She may pick it up, check it or not until later.
My ATF is "old school" where donation is picked up after...  she has a chair placed to recieve my clothes & like at home, I put my pocket junk on the dresser next to it.  The donation is in my key pocket, comes out with my pocket junk & sits there under everything.  When I refill my pockets while redressing, the donation stays behind.  (I did have one young provider who swooped in to retrieve her donation I git the impression she might have feared the donation might accidentally disappear...  Such unscruplious goings on are why many may put it away before proceeding.)
Typically with a Fav lady, the donation will silently disappear during the post after-glow back & forth to the bathroom for both of us.

In short, have it ready, have it correct.
skb

First, check out her specifics on her website and follow them. If there are no instructions, I always drop the donation on her dresser in the beginning of the date and then excuse myself to the rest room to wash my hands. This gives her a chance to check out the donation with out you being in the room. The MOST important thing to remember is to NEVER say anything. I even drop off the donation at the beginning of the session with my ATF's and still say NOTHING. After all, you are paying for her time only.

Good luck, play safe and ALWAYS, ALWAYS respect each ladies rules. Oh, if she knocked yer socks off and you plan on seeing her again, you may want to consider offering her a tip at the end of the session. Tips are never expected but are ALWAYS remembered. Have a 50$ Victoria's Secret gift card handy. They will work just fine. She will love ya for the kind gesture.

SR

As mentioned...

If she comes to you, I have it waiting either in the bathroom or someplace convenient. I definitely have it in the bathroom for her if she is well reviewed. If I am taking a risk, I may keep it elsewhere until I am sure about things continuing past letting her in the door.

If you go to her... I do as SKB does. I empty my pocket "stuff", including her gift, and it magically stays behind when I pick everything up.

BE SURE NOT TO PICK UP YOUR GIFT, which I almost did once.

My only variation from that would be if I noticed VERY explicit gift instructions on her web site.

Most ladies' websites state specifically how donations are to be handled.  Generally, that's at the beginning.

This is all well and good but in a few cases I have had to "re negotiate" what was being paid for.  So not cool.  If a girl says PSE and says $300 on the site that is what I expect.  

I was recently asked for $200 for the extras and then she got all upset I wanted DATY

How do you make it clear what you are getting is all inclusive?  AFTER she has your money you really don't have much choice other than to walk out disgusted.