I’d recommend a position you are both comfortable with and can maintain as you don’t want to be thinking about how being uncomfortable or off balance or whatever. There is no reason why you can’t try a few different positions in one session though, although I would recommend not trying more than 2 or 3 to start with unless you are really struggling to find one that works.
You being on top might be a good one to start with (cowgirl but cowboy?). This will allow you control of the speed, movement, depth etc and will also help her if she isn’t used to using a strap on. Getting a good thrusting motion and controlling something you have no feeling of isn’t always the easiest thing to do to start with and as you are using a strapless strap on, keeping it in is a factor. By laying on her back, gravity isn’t working against her in the same way as if she were on top or on her knees etc. You can do most of the moving to start and as you both become more comfortable she can join in. You will also get good access to her body as a bonus.
If her being in control is the biggest draw, spooning would be another good option. Penetration from behind has more of a being “taken” feel and will give her more control over movement than you on top but it will be easier to keep the toy in place than an upright position.