At the end of this sequence is a guided visualisation, or pathworking, of a ceremony at the well (you may prefer not to read it until your own imagery is fairly firm). It's a nice well, but perhaps it isn't the kind of well you had in mind. That's fine, no offence taken. We're dealing with the brain here, as well as masturbation and intercourse, so the imagery has to be right for you.
Depending upon your tastes and background, you may have in mind a much more formal stone well, a pool within a cave, or even an Indian well in the courtyard of a caravanserai. So, how do you go about building up the imagery? Composing a visualisation that works for just one person is more difficult than it seems: devising one that works for a couple is even more complex. There's no single right way, of course, but here are a few hints that might help.
Your aim is to eventually build a mental image that's comfortable for you both, containing enough detail to taste, to feel, to hold the image through climax. That's a tall order, but taken in easy stages it becomes simpler. Start at the stage of approaching the well. Just let the imagery flow without constraint - the equivalent of brainstorming in a commercial design process. At this stage, record your thoughts separately. Don't push too hard. At first the results will be patchy. Don't worry about this, things will get better as you gain confidence in yourselves and each other.
When to share? Start sharing your imagery as soon as either of you finds parts of the image are becoming important. The imagery will never be quite the same twice, so the very finest detail doesn't matter too much at this stage, but the major elements should match. If the imagery is already completely different by the time you start to share, that's not as much of a problem as it might appear. There's no rule that says you have to have just one well. Work with one and then the other as the mood takes you, but do add detail into each other's imagery.
How to share? That's a matter of personal taste and it'll depend on how you work together best. If you don't already know this, you'll probably find out during the "adoring the rod" and "adoring the graal" exercises. You can work on the imagery simultaneously and then "negotiate" after you've written up individually. This will teach you a great deal about how each other's mind works, because you'll have a complete audit trail in the two journals. You may, however, find it simpler to alternate the development. The recipient of the adoration in each exercise could, for example, work on the imagery and then describe the results to their partner after (or even whilst) writing up.
Your need enough detail to taste and feel it - that's what makes the difference between holding a nice picture in your head, and actually hearing the birdsong as you make love. Equally, though, the detail shouldn't get in the way; gaps will allow for the perfectly natural differences between your points of view. Ensure that there are enough things in the imagery for everything you want to do now, but leave enough space to add new things if you need them. You may just want to meet the Grall Maiden at the moment, but consider leaving a space where you can light votive candles or leave petitions later, if the need arises.
Add the detail gradually. Don't force it: let it build up of its own accord rather than putting it all in at once. If you gently reach a consensus, anything that doesn't work for you will disappear of its own accord and you'll both remain comfortable with the shared imagery. You need to be fairly rigorous in writing up soon after each working and maintaining the continuity, but it's equally important not to crowd the issue by working on it in every waking moment.
What you consciously add in is, of course, a matter of personal taste. Experience suggests, however, a couple of guidelines where, if you break them, you should have a conscious reason for doing so. Think very carefully before adding in a favourite item that might in real life get lost, stolen or broken. As with reliance on tools, it may make a real-life loss even more traumatic.
Its fine to use strong images from your personal experience - an object in your home or a favourite place - but don't make that the "whole" image. If you do, then it's difficult to add in something that's not there "in real life". If you use a pool that you know and love as the basis for a visualisation, consider adding in trees or other elements that aren't there in this reality. You may one day, of course, walk hand in hand round a corner and see "your" place, exactly as you imagined it. But that's different.