Moo moo! We are excited for you to try the new diapers!
There's not a lot that can go wrong when trying diapers on–most diapers we have tried have the capability of tape readjustment, meaning that you can un-tape and re-tape them a few times before they start losing their adhesive backings.
The easiest position to start with is laying down. We suggest that you lay the diaper down flat on the floor or bed, with the absorbent part facing up. Make sure to unfurl the wings of the diaper, but do not undo the tapes yet! Avoid laying on extra plush furniture, such as couches, bunches of blankets or very soft mattresses.
Lay down on top of the diaper, making sure that the front of the diaper is pointing down your body and not up! Would be silly and quite challenging to put a diaper on backwards.
Position yourself in such way that the back waistband is somewhere on your lower back, and that it mostly goes straight across your body. Make sure that the wings are spread out and reachable, and not underneath yourself.
(Optional, but recommended) You can gently pull the wings around your hips and bring towards the front of your body to make sure they are symmetrical. Square yourself on top of the diaper if you're not aligned properly, or if the wings feel like they're too high or too low. Again, do not undo the tapes until you are ready to tape them!
You then bring the front of the diaper up towards your private area, spreading the front wings out and pulling them out and around your hips. Make sure to stretch them a little bit, as diapers need to be snug to avoid leaks and such. The diaper should feel nice and comfortable now that it's mostly around your bottom. The front of the diaper should go up to around your belly button, but can go slightly higher and lower. Reposition yourself if necessary.
The taping process can be tricky, as you have to hold the front of the diaper taught while pulling on each wing. The first tape is the trickiest one, but with experience and learning where the tapes fall on the landing zone will make future diapering a easier.
Start off by untaping the bottom tab (the one furthest away from you) on your dominant hand's side. Then pull the bottom tab around the front and place it down, stretching the wing so it hugs your side. Make sure that there's enough landing zone for the top tab(s). Do not press down firmly on the adhesive, just in case you need to make adjustments.
Follow with the bottom tab on the opposite side. Try to keep the tabs symmetrical. If they aren't, then make adjustments as necessary by untaping and retaping.
Then go onto the upper tabs (the ones closest to you), and repeat the process. You shouldn't need to keep the front of the diaper held taught, as the bottom tabs should mostly keep the diaper snug and in position for you.
The front waistband (if the diaper has one) should be slightly stretched out. We've found that keeping the upper tabs closer to the center than the lower tabs is our ideal fit.
If this guide doesn't make sense to you, we recommend searching Youtube for adult self-diapering tutorials.
Best of luck though!