Junipers are a great place to learn bonsai. They’re very forgiving, tolerant of abuse, but, still make great bonsai. This advice is not a comprehensive guide, it's intended to tide you over until you can consult an authoritative source.
Watering. Don’t allow your tree to dry out. While junipers tolerate dry conditions they do not show water deprivation until it’s too late. The easiest way to establish a watering schedule is to dig a half inch into the soil. If it’s moist you can go another day. If not moist, it’s time to water.
Light. Junipers do best in full sun. Anything less will result in less dense growth.
Pest control. Spider mites are the most common problem. A broad spectrum systemic insecticide works well. An alternative is to spray with horticultural oil, two applications three days apart every month or so. In areas where fungus might be an issue I've found Bayer Advanced Disease Control For Roses, Flowers, and Shrubs to be effective. You can find that at most garden centers. Also effective is copper based fungicide.
Pruning. These trees will lose their style if not pruned on a regular basis. They bud back readily when exposed to full sun. Anytime during the growing season you can pinch off any new growth that spoils the design. About once a year prune more heavily.
Repotting. Every two to three years remove the tree from it’s pot. Trim an inch or so around the entire root ball. Trim off about one third from the bottom of the root ball. Add soil to the pot and around the edges and you’re good to go. You will do this when the tree is coming out of dormancy.
Temperature. Most junipers are hardy at least to zone 4, but, it’s best to provide protection below 45F.
Fertilizing. Any houseplant fertilizer will work. Apply once a month at the recommended level. You can intensify the green color by working a tablespoon of dolomitic lime into the soil. If you prefer a bluish tint you can do the same with agricultural sulphur.
I've been doing bonsai since the late 70's. I'm passing on what I've learned in those 35+ years. These are real life techniques and tips that worked for me, but, may not for you. Remember, this is free advice and worth every cent of it.