There are many Manasa Devi temples in India, but I’m assuming this is talking about the most prominent one, in Haridwar, Uttarakhand. It’s one of the most visited Shakti Pithas in northern India. I can tell what my own visit to it was like.
The Manasa Devi temple stands atop a hill named Bilwa Parvat overlooking the city of Haridwar. Roads go only partway up the hill, after which I walked. On the way up is a Jain temple with exquisite architecture, well worth a visit along the journey.
Then I continued up the hill. The Manasa Devi temple stands high on a forest-covered Himalayan foothill overlooking the city from the west. Here’s a photo I took from partway up, looking down at Haridwar:
As you get close to the temple, there are a lot of vendors selling their wares. Be aware of potential aggressive vendors, sometimes they can be overly pushy. Though at this particular temple no one was inappropriately pushy with me. One guy was selling an absolutely exquisite garland of purple flowers, it was one of the nicest fresh flower garlands I’ve ever seen. You could tell he knew it was good because he didn’t bother saying a word, unlike all the other vendors who were shouting about the quality of their wares; this guy just stood there silently holding up that garland, knowing his product spoke for itself. If you want, you can buy something from one of those vendors and ask them to keep your shoes for you; that’s a lot safer than leaving them in the temple’s official shoe room. If you buy offerings on a metal thali, do be aware that the vendor will want you to bring the empty thali back afterwards, and will yell at you if you don’t.
Manasa Devi Temple was a palpably powerful place. It is said to be the spiritual gateway to the Himalayas, particularly to Manasa’s territory of Garhwal. Manasa is quite known for her temper, and being very demanding of veneration, unusually so for a Hindu deity. She is the Kuladevi of many of the Garhwali people, who are strong Naga worshippers, and she very much demands veneration before approaching other Nagas in her territory. She is said to be blind in one eye.
The temple was bustling with activity, very crowded. Near the entrance is an auspicious tree on which visitors tie threads and make prarthana. It is a very spiritual atmosphere in the sense of crowds shouting devotional mantras and chants with great enthusiasm. There is some nice intricate architecture to be seen. The main sanctum is especially crowded; the compound extends off in sprawling side wings with smaller shrines to other deities which are a lot less crowded. But do be prepared for crowds - some of the time I was literally pressed shoulder to shoulder, back-to-chest in a crush of people. Personally I actually like that vibe - sometimes somebody shouts “Jai Mata Di!” and everyone roars it back in a palpable sense of communal bhakti.
The priests were nice and professional, doing their thing diligently, without making any unsolicited pressures for money or anything.
After going through the innermost main circuit of the temple in a crush of fervent devotees, I was able to find an empty corner (still within the main temple building) to sit down undisturbed for as long as I wanted and do my own puja with a feeling of serenity. I performed a puja to Manasa, following the Manasa Dhyanam from Devi Bhagavata Purana, chanting the Manasa Siddha Mantra and offering flowers, scents, ornaments, food, etc. I recited the twelve names of Manasa: Jaratkaru, Jagadgauri, Manasa, Siddha Yogini, Vaishnavi, Naga Bhagii, Shaivi, Nageshvari, Jaratkarupriya, Astikamata, Vishahari, and Maha Gnyanayuta for protection from snakes, and read the Stotra of Manasa.
I also did a puja for her sister Mangalachandi, reciting her Dhyana Shloka, Mantra and Stotra from the Devi Bhagavata Purana and so on.
There also may be opportunities to participate in temple activities like volunteering in the kitchen, if you’re interested. Every day the temple serves bhandaras (free meals) to all visitors if you go at mealtimes.