I was in the same boat when I first started, very hard. I found to wet them, just either drink lots of water fast, or if you want to start flooding them (and you're old enough to legally do this) then drink lots of alcohol, along with water between glasses. It really makes you have to go, and if you can get comfortable and in private, you can empty your bladder
For messing, first time I started doing it I did it when I had to REALLY poop, and first started in panties, since was too afraid for diapers. Then recently found the joys of suppositories. Marshmallows, glycerin, ducolax, ect, makes you want to push it into your diaper.
Feeling wise, for me it's that shock of loss of control, helpless feeling (I get that when I use suppos as soon as they're in, since I'm in for the long haul). and it's really hard to explain. Depends on why you like being in diapers personally.
Changing yourself isn't that hard. I have a towel under me, and lots of TP, and more recently wipes. Flushable wet wipes, or just baby wipes really work, then I finish off with a shower using a poof thingy and lots of soap around that area and anything that was covered in the diaper. If you're able to find an hour to yourself in the bathroom, you could quietly fill yourself up, diaper up, mess and relieve yourself, then clean up and shower after you get the rytheme. Probably pricey to find someone to pay, and good amount of time hunting to find someone nearby with interests