Get some 75mm cold tape along with the solvent (I presume that we are talking of a butyl linner) from this company. http
://www.absolute-koi.com/subcat188.html" onclick="window.open(this.href);return false;.
The liner should overlap at least 75mm on any seams and any corners should be mitred, if you have a problem with this procedure practice on a piece of paper first. In fact build your pond as a model in paper. If you do this you can practice you seams and look at any problem areas before you start, use stick glue and don’t forget to apply the corner patches to you model, this is a crucial part of making a good box liner.
Butter both sides of the seam with the solvent and let this dry, DO NOT LET THE SEAMS CONTACT BEFORE YOU ARE READY. Apply the tape to one side of your seam (you can do this in small strips of cold tape if you want but you must make sure that it slightly overlaps to ensure a seal) and roll out the air bubbles, then place the opposite piece of liner with the buttered side down to the tape then slowly remove the tape protective cover this will then allow the seam to become one, make sure that you roll out any air bubbles again.
The corners are crucial and so once these are completed I would cover each with an extra patch to ensure that you have a 100% seal.
If you want to make doubly sure, get some spare butyl and cut some 75mm strips. Apply the solvent to the pond and to the spare strips. Once the solent is dry apply the 75mm cold tape, smooth out any air bubbles the apply the butyl strip and again smooth out the bubbles. Easy, oh by the way use a roller to make sure you have no air bubbles, this is quite easy if you use the approved roller, although the tape sticks like sxxxt to a blanket it will still work very easily for some time after fixing.
Don’t rush, you have all the time in the world, you can still work the seam with the roller for ages.
Any probs drop me a PM, I will be out of the country for a few weeks after Friday but I will get back to you ASAP.
Phil