This general scenario guide provides a very generic walkthrough of most RollerCoaster Tycoon scenarios. Guides for specific scenarios can be found on this page.
Most RollerCoaster Tycoon scenarios automatically open the park details window, set to the scenario objectives tab, when you first load the scenario. You are also strongly advised to pause the game at this time so that you are not distracted by anything else happening in the park.
If this is your first time playing a scenario, read the objective to get an idea of what you must do in order to successfully complete the scenario as well as how much time you have to reach said objective and, if the scenario's author bothered to write a Chris Sawyer-ish park description, any scenario limitations in effect and/or specific details relevant only to this scenario (e.g. "...won't allow any landscape changes...", "...badly in debt...", "...land is cheap but loan interest is high...", etc.). Next, scroll around the map to get a general overview of your park. Things you may wish to take note of at the start include any existing rides in your park, your park's boundaries, the scenario's loan limit and interest rate, and the number of rides already available to you for construction.
After that, identify any problems already existing in your park, such as excessive litter, lost guests or unsafe or broken down rides. Although you cannot clear litter with the game paused, you can hire and place handymen so that they will immediately start cleaning up any mess already existing in your park when the game is unpaused. Lost guests can be picked up by you and placed on footpaths close to the park entrance. Any existing ride in your park that you feel is unsafe can be reset by closing it twice. If there are already broken down rides in your park, hire 1 mechanic (if there isn't one already) and place them near a broken down ride—they will set about fixing any broken down rides once the game is unpaused.
Finally, if your park has any rides already running, hire 1 mechanic (if you haven't already done so in response to a broken down ride). For the time being, this mechanic does not need to be set a patrol path, since they are only a temporary measure to handle any (additional) ride breakdowns while you set up your park. You may also wish to take note of what kind of rides these are (i.e. transport, gentle, roller coaster, etc.) as well their statistics (such as their ride rating, admission fee and queue time) and, for tracked rides such as roller coasters or go-karts, their size. At this point, it may be necessary to unpause the game and modify some pre-built rides or their queue lines for more desirable ratings.
Unless certain conditions in a scenario create the circumstances, your park should have a mixture of low- and high-intensity rides to cater to as many guests as possible. A park that is starting out will generally have 1-3 gentle rides, 1-2 thrill rides and 1 roller coaster, so build the first new rides in your park accordingly. If the park's geography permits, build your first few rides on flat land, near the park entrance, with your least intense rides being placed closest to the park entrance. In addition, try to place the ride exit for your rides such that they connect directly to the main pathway from the park entrance. It may even be beneficial to relocate the ride entrance/exits of existing rides to facilitate guest movement around the park.
While you are expanding your park by building new rides, you may need to construct additional footpaths so that guests can get to your new ride. Try not to construct long footpaths (about 5 tiles long or more) that lead to a dead-end or ride exit as guests are likely to get lost along these stretches.
Reaching the Objective
Every once in a while, review your scenario objectives—it is very easy to lose track of what you are supposed to do during a scenario if you are busy managing and/or expanding your park. If the objective is to reach a high park value, consider building more rides to attract more guests; if the objective is to have a large number of guests, you can also consider taking out a marketing campaign or two if the scenario allows it. Keep an eye on the date as well; the results of anything you do will only be seen after a period of time. For instance, if your objective has a deadline of 31 October, Year 3, constructing many rides or starting a marketing campaign on 26 October, Year 3, will not miraculously cause your park value or the number of guests in your park to skyrocket within a few days.