You should be measuring your doses with a milligram scale. You may know that already, but you didn't mention it so it needs to be emphasized. Some people will take the weight that the vendor specified at face value and then use liquid solution measurement in an attempt to ensure they can dose accurately if they lack a milligram balance. But the issue is that the vendor may have decided to be generous and put in 700 mg instead of 500, or may have been a shady, skimpy fuck and put only 400 mg in. So it is critical that you be able to independently verify the weight of the total parcel, as any deviation from stated weight will ensure that your doses are not accurate even if you do use liquid measurement.
Once you have verified the total weight, you can dissolve it in a given amount of liquid so that you can measure out doses that way. This would be the ideal if, say, you only have access to a milligram balance by going to a friend's house who has one and measuring it there. If the weight is indeed 500 mg then you could dissolve it in half a liter of liquid, so that a milliliter will contain a milligram. Different ratios of liquid to weight can be used, so that you don't need to drink as much foul tasting liquid (though the lesser volume will itself taste more foul due to higher concentration, there will just be less of it).
If you purchase or already have a suitable balance yourself then you can keep it as a powder and measure your doses by weight. This is probably the most stable way to store it.
I would suggest taking it orally after the proscribed allergy test (taking a single milligram to ensure you don't have a weird idiosyncratic reaction to it), and seeing how you like it. If you like it then you can explore other ROAs from there. Plugging isn't my thing, but after oral dosing, it would probably be the next best option, and you may even enjoy it more that way due to faster onset and increased potency due to higher bioavailability. Intranasal may be an option, but generally RC psychedelics do not feel good in the nose. It may not burn like PEA hydrochloride salts do, but I doubt it will be pleasant.
Other than choosing a route of administration and ensuring you don't overdose by using a milligram balance, there's not much else to say. This stuff is not as potentially mindblowing and powerful as some of the more traditional tryptamines like psilocin or LSD, but that makes it no less interesting or valuable. Personally I haven't tried it yet, so I cannot give firsthand advice, and so you can obviously use google, erowid, and the forum search to research more about its qualitative effects.
Do not eyeball or estimate doses. You could kill yourself in theory, though I don't know of any conjectured or confirmed deaths from overdoses of 5-MeO-DALT, or seriously harm your body or mind. I'm not trying to talk down to you here, as I said for all I know you've got a $5000 submilligram balance stashed in your room, but just in case I'm reiterating this point yet again. It can't be emphasized enough, so be safe and thusly enjoy this compound without fear of overdose.