To cut the steerer tube is not a job for untrained people, so let a professional mechanic install the fork.
See manufacturers installation guide for the headset and handlebar stem you are using! The SR SUNTOUR steerer tube may need cutting to the proper length. The SR Suntour suspension forks with hydraulic system are available only with threadless steerer: choose the right headset for this Ahead type of steerer. The steerer tube must be cut to proper length correctly Special tools and skills are required If you install the fork by yourself, the completed installation must be inspected by a qualified bicycle mechanic and signed off to validate your warranty. INSTALLATIONSR SUNTOUR recommends that your suspension fork be installed by a qualified bicycle mechanic, because it must be properly installed, adjusted and checked for safety. The use of other replacement parts voids the warranty and could cause structural failure to the suspension fork. In these cases, take your bike to a qualified bicycle mechanic for inspection and repair. 2.) Do not ride the bicycle if you notice bent or broken parts at the fork, especially after a collusion, sounds of excessive topping out, or other indications of a possible fork failure. If the brakes do not work properly, the rider could suffer serious injuries. IMPORTANT SAFETY INFORMATION1.) Before riding the bicycle, be sure the brakes are properly installed and adjusted. Suspension forks can dramatically improve the comfort and control of your bicycle, but they can also change the way it handles, so always exercise caution and good judgement when getting used to your new setup. One of the things you may notice, is a small amount of play in the lower tubes. During the first 300-500 km, the ride dynamics of the fork may change slightly. As with any new suspension fork, there is a short period within moving parts will break in. When transferring ownership of the suspension fork, give this instruction sheet to the new owner. After reading this instruction sheet, please be sure to keep it in a safe place for future reference. The instruction sheet contains important information on the correct installation and usage of the fork. Be sure to follow all warnings and caution indications. Using the suspension fork inappropiately may cause serious injuries. Owners ManualSuspension front fork with / or systemRead this instruction sheet thoroughly before using the suspension fork.