I have been exactly where you are now and I understand what your saying as it can feel strange or even lonely to have to do this by yourself however as there are many pepole like you and me don't worry too much.
Firstly could I ask you a question do you have the internet?, as both this website as well as wikihow and Adisc.org and many more sites are my best choice of instructions although be careful not to accidentally access any porn sites whilst searching online.
However all that being seid if your out and about and happen to purchase a pack of diapers just follow the instructions on the back of the package as most have a pictorial description, just look up how to put on a adult diaper.
If you get pepole asking why do you need diapers just casually say you have a disability or medical need to wear them and this should surfice you never know you might be offered help or assistance from a kind person but be careful around strangers unless your confident you won't be harmed.
However if your still struggling I would suggest you start by opening a pack unfolding the diaper or nappy put the tabs to the back so your facing the side without them put the diaper on a flat surface sit yourself in the middle of it then hold onto the back with one hand whilst bringing the front part up between you legs then hold onto the front part.
Bring the sides without tabs or sticky labeled into the sides by tucking them into the sides of you hips so they are comfortable readjust if needed then start either with the left hand side or the right this is your choice, then bring the back panel with the sticky tabs into place stick the tables on top of the side you just tucked in and apply the bottom one first about where your crotch area is then do the upper on where your waist line is and then start on the other side and repeat the process,
Personally speaking I find either sitting or laying down to be the best option, however if you need to apply the diaper standing follow the same process.
If you need more instructions I suggest you either get help or if that's not an option go to YouTube or other online sources but again a safety tip is always be careful not to access any porn sites as they can pop up if you are looking online my best tip is make sure your incognito as in private so pepole can't access your history online also delete your internet history if you are aware how to use a computer.
Ps- here's wishing you a Merry Christmas and new year as it's that time of year again.
Yours sincerly
Chinababy888.