Q.How far apart should Forsythia be planted? Do they like shade or sunlight better?
This is my first time to have a Forsythia. Do they prefer more sunlight or shade? What’s the correct procedure to plant it?

Forsythia requires at least 4 hours of direct sunlight to bloom but full sun, 6+ hours, leads to heavier blooming. It has been a garden staple for so long that there are many hybrids of different sizes. Therefore, go with instructions on the plant tag. You want to leave room between plants so they can take their natural arching form. That may mean 6 feet apart unless you are using them as a hedge. After blooming, remove 1/3 of the branches to the ground annually starting next year. Remove the largest stems first. Forsythia blooms better on young wood; by removing thick, old branches you are allowing fresh growth to thrive. Flower buds are made the previous year in late summer and fall; therefore, pruning must be done before July 4. Where branch tips touch the ground, roots develop and start a new bush. To prevent crowding, these should be removed or transplanted. The following article has more care details.
https://www.gardeningknowhow.com/ornamental/shrubs/forsythia/forsythia-shrub-care.htm