If you have a mac, set your keyboard to U.S. Extended. Then, to type a letter under a tone, press option + (following key), and then press the letter you want the marking to go on top of.
n = ˜, 6 = ˆ, b = ˘, e = ´, (key left of 1) = ` , x = . , z = ˀ , and i = ʼ Ex: ã, ô, ă, á, à, ọ, ủ, ư
To type two symbols on a character: First type the character with the symbol that is underneath. Then press option + shift + (the key for the other symbol) and it will put the other marking on top.
for ẫ you would type option + 6, then a, and you'll have â. Then you press option + shift + n and it will show as ẫ.
I think you can also switch your keyboard to a vietnamese input, but I'm not sure how to use that at the moment. I believe there's a discussion on it somewhere on Duolingo.
I don't know about Windows computers.
I guess if I need to respect some typing order, for example when typing ẫ (using the MS OS Vietnamese keyboard, it should be this: firstly, key 2; secondly key 7, no problem with this; and using the Telex input method, I think it would be: pressing twice the key a, and then the key x; but perhaps someone prefers the sequence a-x-a).