Since I wear cloth, I first spread out and layer the ones I'm going to wear on the edge of bed, the front 1/3 of them hanging over the edge, positioned so that when I sit down on them my butt is in the right position with respect to the diapers.
This usually involves at least three layers--a relatively thin Gary contour flannel diaper on the bottom (will be on the outside), a big thick bulky flannel prefold--folded lengthwise into thirds--in the middle for a soaker (and if I know I'll be wearing / wetting them a lot or just want them to be extremely bulky, I'll do two of these), then on top a nice, thick soft flannel contour that will go against me. I then get two big adult-sized pink ducky diaper pins and pin the outer contour to the inner one in the back but leave the pins open.
Then, standing up, I cover my entire diaper area with pink Baby Magic lotion (or Johnson's if that's what I have at the time). Then I sit down on the diapers (careful to not get poked by the pins!). Then I pull the fronts of the diapers up between my legs, over my lotion-covered private parts (OMG this part feels so babyish and wonderful!). Then comes the pinning--which is the worst part.
Since I'm right handed, I first roll over onto my left side and pin the right side of the diapers by pinning the back corners of the two contours to the front of the outer contour about at my waistline. Since it's the hardest side for me to pin (since my left hand has to do most of the work) I do this side first when I have lots of "slack" to work with.
Then I roll over onto that side and pin the left, easier since I"m reaching over and using my right hand. I try to get this side even with respect to where I pinned the first side, and as tight as possible (knowing that since they're cloth, once they get wet they loosen up quite a bit). After years of experience I've actually gotten pretty good at this.
If I'm going to be going out in them, I'll usually add a second set of pins for "security", below the other ones, near the bottom of the diapers just over my legs. Those are fairly easy to pin on and I do that while standing up.
Then I pull up my plastic pants. That's probably the best part because the plastic pants going on is (for me) one of the most babyish aspects of all of this--and the more babyish the plastic pants look, the more fun this is.
If it's a hot day and especially if I'm going out, I'll go stand in the shower (so it doesn't go everywhere) and put some powder around the waist / leg holes of the plastic pants.
I've been doing this now for about 50 years and have actually gotten pretty good at it. Getting older and having a big stomach does not make this easier! Recently I ordered a few new, larger outer flannel diapers that are wide enough to make this easier. However the easiest way is to have my wife do it, and sometimes she will but if she's on the sofa watching TV I hate to ask her. That, or our kids are over here visiting and that makes it impossible. Oh well.