How everybody likes their undies to fit is a pretty personal thing, so in this post I am going to show you some ways that you can alter my free Boys Trunks Pattern to get the fit that you want.
For a looser fit in the legs
Slash the main pattern piece through the leg, up to the crotch stitching line, as shown in the photograph below. Spread the pattern piece the desired amount. Smooth out the leg opening curve when you are cutting.
For a looser fit in the crotch
Take the crotch pattern piece and make two slashes, from the centre front through to the stitching line on the other side, in the bottom half of the crotch piece. Spread the desired amount. Again, smooth the curves when cutting out.
For a closer fit in the crotch
Slash the crotch pattern piece as shown in the photograph for a looser fit. Then, instead of spreading the pattern pieces apart, you overlap them the desired amount.
To change the rise (height of the waistband elastic)
For this, you will need to make alterations to both the main pattern piece and the crotch pattern piece. Sit the crotch piece on top of the main piece, so that the sewing lines (not the cutting lines) are sitting on top of each other and there is a smooth line at the top of the pattern piece.
You can either cut the top of the pattern pieces, to make a lower rise, or add some height to the pieces to get closer to the waist. You do not have to remove / add an even amount all the way across. In the photograph below, I have lowered the rise at the front of the pattern, but left the back rise as it is. This is shown by the pink line on the pattern pieces. Just make sure that the back line is at right-angles to the centre back fold line (CB) and the front part of the line is at right angles to the centre front (CF) seam on the crotch piece.