it takes a lot of practice, hehe.
remember to make your "mental checklist" and just go over and over it while you are practicing
...make a probe...
...check scout probe...
...build a unit...
...check sout probe...
...chrono boost...
...check scout probe...
...build probe...
...check scout probe...
...build a unit...
etc, etc...
just form that check list and work on 1 or 2 of your mechanics, like keeping the scout alive while building only probes until time runs out.
then next game, keep scout alive, build probes, and make 3 gateways
next game; keep scout alive, build probes, make 3 gateways, make units
next game; keep scout alive, build probes, make 3 gateways and assimilator, make units
you get the idea.
eventually you will be able to do it all, no problems!