where do you put custom lots?

Eyeball the open areas available where you'd like a new lot... which are ALL connected to roads, no placing lots without road access... choose the lot size you'd like to place, pull it out, noting the dotted road outline with arrow pointing to front of lot, rotate template so arrow points toward existing road, place road outline over existing road, then click to place. If the lot is too large, it won't place. Before you finalize, make sure you choose the type of lot it will be, Residential or Commercial. Then OK placement.