So you've got a dictionary in your back pocket for emergency reference, and you've spent a few days just taking in the flow of conversation around you. You have a general feel for what's going on, even if you don't catch every word. You're ready for one of the most harrowing and yet rewarding experiences of an immersion education — speaking.
It's simple: You should try it. Just open your mouth and let the words come out. Accept in advance that, yes, you may sound different from others around you; yes, you may pronounce a few words wrong here or there; but your goal at first is not so much to have perfect diction and grammar but to make yourself understood. Once you're capable of doing that, you're ready to fine-tune your communicative abilities.
Again, resist the urge to clarify things in English whenever possible, unless you're able to switch right back into Spanish. Some people, especially in other countries, will seize the chance to practice their English on you instead of allowing you to practice Spanish. Instead, ask for clarification in Spanish. So instead of asking, "Cómo se dice 'horse' en español?" you could instead query, "Cómo se llama aquel animal?"
Before you know it, you´ll be ready to abandon the dictionary entirely. After all, that is the point of Spanish immersion.