I know that easy way is organise a permit, go up by cable car and walk 30 minutes up to see the top or spend some additional money for tour with local company. Some people chose altavista night and hiking up but i suggest even better way, most adventurous. Just do an overnight hike up from Montana Blanca parking. We left from parking shortly after 0:30 as all guides indicated that it takes up to 5,5 - 6 hours to get to the top but with decent pace you can do all of it in 4 - 4,5 depending on your fitness level. Just get some light source, warm clothes and you won't be dissapointed - it is totally doable, but temperature and wind conditions can be harsh up on the top. Sunrise was spectacular, no permit required for volcano and no booking for altavista so that is the perfect solution for people coming to Tenerife with last minute offers and with no figured-out plans. We got down with cable car even that it was not taking any people up (closed due wind conditions)