Drop the PDC parts on the planet, drop the engineers on the planet (or the engineers first, works either way).  F2 screen, Industry tab, select Assemble PDC, select the PDC to be assembled, quantity 1, percent 100 (or whatever you want).

Verify the PDC parts are in the Stockpile

I'm pretty sure it won't let you create the job if the parts aren't there and once you create the job the parts are removed from the stockpile. (I guess it's a bug that if you cancel the assembly the parts aren't put back into the stockpile.)

Drop the PDC parts on the planet, drop the engineers on the planet (or the engineers first, works either way).  F2 screen, Industry tab, select Assemble PDC, select the PDC to be assembled, quantity 1, percent 100 (or whatever you want).

Verify the PDC parts are in the Stockpile (F2 screen, Industry tab) and the engineers are on planet (F2 screen, Ground Units tab).  Verify you have the correct minerals on planet (takes about 10% of the minerals used to make the PDC).

One Engineer Brigade will take just short of forever to assemble a PDC, I tend to use a division or two.

One thing to note, I've seen bugs with PDCs at every step of the process,  making the parts, transporting the parts and assembling the parts.  Sometimes I go through the right steps, and the PDC vanishes.  In those cases I've SM'd a PDC onto the planet, and then after that is mostly works.  Assemble the first one at that colony, verify it worked, then queue the others.

Latest version, I've dumped engineers on worlds and had them assemble PDCs, and done the reverse, dumped the PDCs down first, then brought in engineers.  Haven't needed colonists.

I have definitely assembled PDCs using only engineers on colonies where I had no construction factories.  Maybe it is necessary to have the engineers in place before setting up the task?

?  thats weird-- I have 5 engineer brigades in place and have the expected industry..............

So OK how do you assign an Engineer BRigade to assemble a PDC - all I managed to do was que it as an industrial project which I don't have any capacity for and then when I cancelled that I lost the PDC completely.