This is a subject I've had alot of experience with.
For beginers, Mr. Big is on the botttom a Mr. Reciever sits down on him giving Mr. Reciever total control of insertion, speed, depth.
Secondly, Mr. Reciecver lies face down and Mr. Big inserts. No truly "dogie" but a variation. I have found this position a little easier for Mr. Reciever.
Thirdly and maybe most important Mr. Big needs to be aware of his size and go slowly for the first month or so.