Often your cutting is not as accurate as you would like so there are small gaps between the pieces as you assemble the piece.
One solution that is often used is to grind the edge of the too large piece to get the fit desired. The problem with this is that thorough cleaning is required to avoid devitrification lines appearing on the final piece. Also, even with extensive grinding, the fit is not perfect.
The alternative is to fill the gaps with fine frit or powder.
Assemble the whole piece and assess the gaps. If they are very large, you need to adjust the glass. If they are only millimetres wide, powder and frit can fill the gap to disguise the join. I generally use powder for almost perfect joints, and fine frit for anything larger.
I first cover the gap with powder or frit and with a soft brush work at right angles to the line of the join. This ensures that I have filled the gap to the height of the glass. However, the frit and powder have air spaces, and so will fuse to a lower level than the height of the glass. So, once gap is filled, I build a small ridge over the gap trying not to extend beyond the gap. This mound compensates for the lack of density of the frits.
The frit and powder colour must match the glass exactly to become invisible. It can be made from your scraps or purchased at the same time as the glass. I find it more successful to do these fills with the darker glass. It provides a more distinct edge to the joint. It also conceals the base glass better.
It can also be used to conceal the joint in a single colour where the piece cannot be cut as one and needs several pieces to make the whole. This is more simple as any overspill will not be noticed when fused.
This method only works with full fusings. At tack fuse temperatures the frit will not fully combine with the sheet glass to form a smooth invisible join surface.