Hmmmm Well, No. That's not exactly the advice I'd give.
Find out how you floor is constructed. Unless it is EXTREMELY old it will not simply be a single layered planking job over stud walls. You need to know how thick it is. How you are going to retain the structural integrity of the area surrounding the hole. How you are going to reconstruct the plug so it has structural integrity. Determine the Bevel/Swing of the plug so that it's downstage side can be properly beveled, this depends greatly on the thickness of the floor at the edges and the type/wieght of hinges used for supporting the upstage side.
Personally I'd have a structural engineer look at it first prior to making any major modifications. <believe me not matter how small a trap, this IS a major modification.>
There are a a ton of things to be considered. If, for example it is a "sprung" floor, or a floating floor, you are going to have two layers of 3/4 - 5/8" ply on the bottom, layed over each other at right angles. then you will have a layer of 1x6 - 1x4 "sleepers" sitting lying on face at 45 degree angles to the top layer of ply. Then you have a layer of 3/4-5/8 and sometimes 1" T&G plywood running up to downstage as the topmost layer then your masonite cover or whatever.
Now if you didn't understand everything in that last paragraph, don't even attempt the project yourself.