The MZ wheels were photographed to have a reference for the spoke layout (which is pretty self explanatory anyway due to the way the hubs are drilled). They were relatively easy,as they didn't have any offset & the spokes are sold in the correct length. I did the truing by putting the wheel on a wheel spindle, holding the spindle in a Workmate & used an engineering surface guage to hold a pointer as a fixed reference point.
When the wheel was running true (better than half a millimeter on the side & OD) with the spokes snug-but-not-tight, each spoke was gently tensioned in turn until all "pinged" at around the same pitch while keeping running true. As the spokes were the correct length, no trimming of excess length protruding beyond the nipples was required. Same goes for the thread showing - with the correct spoke, very little if any thread shows.
The bicycle wheels were a bit more complicated, as I got the hubs & rims new, so had no existing spokes to refer to for length. I used an on-line wheel building calculator to work out the spoke lengths for the "cross 3 " lacing pattern I decided on and to suit the dimensions of the hubs & rims I had. And it went suprisingly well, using another on-line wheel building site that explained the order in which to build the wheel. IIRC I had to trim the spoke ends with a Dremel, but not much if any thread showed below the nipples. I used the bicycle turned upside down to hold the wheels for truing, using a black marker pen held against a fork leg to highlight the runout.
It helps to have a good quality, well fitting spoke key. Otherwise, just work methodically & take your time. If it starts going awry, back off & try again - don't force anything and nothing should get damaged. If you really can't get it sorted then a wheel builder should be able to use the parts to finish the job if you haven't mauled them.
Go for it !