Diphenhydramine is literally the most fucked up drug I've ever done (and I've done basically everything).
Aside from the fact it's not good for you (which as drug users, I think we've all resigned ourselves to a bit of danger-- everyone's line is different)
Doesn't matter how much experience with tripping you have, as (unless you've done other deliriants) nothing can prepare you for a benadryl trip.
For me (I weigh 140) between 400-500 mg is plenty to make me trip balls.
I don't know what other drugs you do, your tolerances, or your tolerance to diphenhydramine.
But if you smoke weed I suggest you do that about 25-35 min after taking the pills. It goes a long way in the hallucinations (which are true hallucinations BTW, not like LSD or Shrooms)
Idk if you have access to/do opiates, bit a light dosage enhances the trip quite a bit.
You just have to be careful and know how much you can handle so you don't get overly depressed breathing.
All in all I'd say do this:
T+0:00 - take 350mg within 10 minutes
T+0:15 - take a LIGHT opiate dose (I use bupe personally, but if you don't have an opiate tolerance take like 5-10mg Hydrocodone... Or whatever is a similar dosage for your tolerance... This step is optional but it helps)
T+0:20 - Drink caffeine, it will enhance the trip and help you stay awake (just don't overdo it, for me I drink about 100mg which is half my normal daily dose)
T+0:25 - smoke weed (optional but highly recommended if you want to have a lot of hallucinations)
T+0:40 take another 100mg diphenhydramine (repeat if you aren't tripping in the next hour + a half)
T+1:00 - smoke more weed if you wish.
And from there you should trip hard-- I'd highly recommend a sitter but I never had one.
Probably best if you have them do their own thing and just keep an eye on you as you'll be in your own little world.
Also make sure to get several bottles of water ready to stay hydrated.
If you smoke cigs I'd have some, but also get snus or something smokeless that you can put in and forget, as you may burn yourself/drop cigs, etc.
Don't expect visuals to be "distortions", you will straight up see people/animals/things that are not there.
I've hung out with people for hours just to realize they were never there. Prepare for a mindfuck.
Personally I'd advise you not to take more than 600mg. If you don't trip with this method by 600, you may as well give up trying. Anything above 600 is far too dangerous for me to recommend.
Also try to stay awake for like 5 hours.
For me it takes about 2-2.5hours to peak.
Anyway it's not for the faint of heart, if you trip there will be times that you will be very uncomfortable or scared. (Opiates will help with that though)
It's very intense, but I don't regret it and have done it several times.
Do some research and above all be safe.